Since Cesare interacts with the AC cast a lot in Camp, he has found out a decent amount about his alternative universe self. And what he has learned... rather disturbs him, to be honest.
So Cesare from Assassin's Creed is more ruthless and, eventually, madder than Cesare in Camp, which would not be concerning except that he does consider them to be the same person, to an extent. It's not that he thinks he is exactly the same as this other Cesare so much as that they are on the same path and have very similar influences in their environments and, potentially, did start off as the same person. He is willing to entertain the idea that the two of them are the results of slightly different choices from the same core beginning and to that end he does relate to this other Cesare as another version or aspect of himself. They are the same/not-same and if nothing else AC!Cesare holds up a mirror to Cesare in Camp and enables (or forces) him to take a better look at himself.
Cesare's biggest drive in Camp is to learn as much as he possibly can about anything that he might need to overcome the history that has been written about him everywhere he looks. He arrived in Camp in late 1491, which gives him sixteen years between obtaining his degree and winding up dead in a ditch in Navarre. He's sixteen right now. He is literally having a mid-life crisis and it is driving him to seek out answers wherever he can, be it from books or people or Camp itself.
Ezio was the first person Cesare met who recognized him, but this was not interesting beyond "oh, look, another person who wants to kill me, wonderful". Shaun was much more immediately intriguing to him because Shaun was clearly from the future and yet he too knew who he was from the start and, more than that, knew about him. This made Shaun incredibly dangerous and also (if he could convince him to work with rather than against him) incredibly useful. Since Cesare's dealing with other versions of himself and his family (not just the Assassin's Creed canon, although that does come up a lot) ties in very closely to his dealing with Shaun, I'm just going to quickly give a breakdown of that CR. :|b
meet -> mistrust, danger -> truth post, haet, plans to kill -> decides instead to manipulate into closer contact -> spending more time with him as part of that plan, talks to under alias, goes through laptop, etc -> I believe the term coined for this was "douchemance" (-> for about a week start to forget about plan to manipulate) -> accidentally crazy and murder -> ohshit why happening D:
What Cesare wanted out of Shaun has always been information. He wanted to find a way to change his future and he recognized Shaun's potential for helping him to that end. He wanted to get close enough not so much to find out more about the Assassins and their own weaknesses--he's figured out that they don't matter in his own world--but to learn about the Templars or, more specifically, the Borgias. Cesare has been hunting down anything to do with his family, he's read or watched just about every adaptation of their story, regardless of the fact that it is incredibly painful for him to do so most times (sob Showtime sob Cantarella sob). He needs to know and to understand the mistakes made and he is convinced that Camp has given him the opportunity to do so. Family is hugely important to him and he has been given a chance to save them from their fate. This is really not something he can stop working on.
And that gets him into a strange situation with Shaun. While on the one hand he would like to give him more information about how things and people really were, to correct some of the misinformation and to task Shaun with spreading that truth, he also cannot fully trust him. Shaun has made it very clear that he is working against the Borgias back in his world and Cesare understands and respects the loyalty Shaun has to the Assassins. At the same time he cannot make as clean a break between the Borgias in different universes as Shaun can because they are still family. They might be crazy family, they might be those relatives you don't want to talk about in polite company, but the importance of family has been drilled into him all of his life and at this point in his life at least it is still very much that thing you can always fall back on and rely upon. You do not turn your back on family, especially not on the core unit. Shaun was right when he said that Cesare would instantly align himself with another Borgia from a different world. It's an automatic thing for him, a defensive reaction he has learned to immediately go to. It could be broken--but it would take a lot of effort and a sufficiently dangerous reason.
Because of this, Cesare does not feel comfortable sharing more than the bare minimum about his inner circle with Shaun. Anything he tells him will get back to the other Assassins and will get used against those very people who are the most important to him, even if it is only against their counterparts in another world. Cesare sees Shaun as a mole and he cannot bring himself to fault him for it but he can refuse to give information. It's a power thing and, as stated before, Cesare has Issues about power. He will only talk about his inner circle when he absolutely needs to, when he feels that the very foundation of CR with Shaun is about to give way and when he needs something to shore that up. Even then, it's a piece of information here, something totally unrelated there, and he has to get something back that he considers to be of greater or equal value. The thing of greatest value to him right now would be information on AC!Cesare himself and on his insanity.
After the Higurashi event in particular, Cesare is afraid that he is harboring insanity within himself and that it will one day overcome him. Insanity means loss of control and an inability to push forward with the mission at hand. It means being unable to see or to take the steps that are needed and having the fault for the failure of the creation of a Borgia dynasty fall squarely on his shoulders. He needs to not go mad or, at the very least, he needs to understand this madness and to take measures to counter it. During the Higurashi event Cesare lost all control even as he exerted it over Shaun. Looking back over those days scares him because they could very well be a portent of things to come. While his initial knee-jerk reaction many months ago had been to kill Shaun, Cesare had recognized that as rash and coming from his anger. He had known it was not the best solution to his problem and so he had scrapped the impulse and worked instead on building up a better relationship between them. He spent a lot of time appearing (or, well, seeming to appear) immature, rash, innocent. The goal was to have Shaun see him as Cesare first and a Borgia second and for the most part he succeeded, perhaps too much so. This was a long-term plan and it took several months to play out. Towards the end of it he started to lose sight of what its purpose was. Cesare has a tendency to calculate down to the very last detail but also a tendency to overestimate himself or just to forget to add the emotional side into the formula. Of course he planned on having Shaun bond with him. He just did not foresee that the feeling would be mutual.
During the months that Cesare was toying with Shaun, mocking him, going through his things, every so often dropping a very genuine compliment to reel him in, he was slightly shifted away, ready to turn and to run at a moment's notice. Shaun was a stepping stone in a larger plan and if he could use him to get to that information he so wanted it would all be worthwhile. It was hard at first to behave so naturally, to actually act his age for once, but he felt it would be worth it in the end. This plan actually ended up influencing a lot of Cesare's other CR since he had to hold up this act around other people as well in order to make it convincing. It could be pointed out to Cesare that going through all these steps might in and of itself be an indicator of the sort of madness he is recoiling from, but fortunately no one has brought that up to him yet, haha. For right now, this plan was part of a very necessary defensive maneuver and his suddenly breaking with it was a mark of insanity.
It was Camp, of course, that forced him to do so, but it still hit too close to home considering that this was a previous fear of his. Convincing Shaun that it was Camp and not his own idea was so important to Cesare because in doing so he would be able to convince himself of the same thing. He was just a vessel! This isn't something he needs to worry about back in Italy! He will not go mad, he will retain his hold on the Romagna if he just tries hard enough...! After that taste of insanity he needs more than ever to learn how to defeat the monster he is afraid might be stirring within him. He wants to recover that position of relative confidence he held with Shaun in order to learn more about AC!Cesare and he is starting to realize this is complicated somewhat by the fact that he also just honestly misses having him as an almost-friend. Shaun is one of the only people in Camp who knows enough of the background information that he could potentially understand specific frustrations and desires Cesare has. Cesare very honestly feels the need to mend their relationship both because he needs the information Shaun can give him and because he was just coming to rely on him for something more.