Not really following this whole council serial killer storyline as well as I could be, but here's what's happened so far as well as I can follow.
Victims [all founders council members]
- medical examiner [killed]
- Bill Forbes [killed]
- Alaric [halfway killed]
Named Suspects
- Klaus [& Tyler]
- Meredith Fell
Clues
- The weapons all belong to Alaric, so anyone who knows where they are and has access to them could be responsible. (This includes: Elena, Jeremy, Meredith, Damon, Alaric, and we'll assume others in the "group")
- Elena's fingerprints (Which means Katherine... and theoretically Tatia, if she's not dead and gone, could also be suspects) -- however, that could be explained by Elena using Alaric's weapons for training.
Random Stuff I Noticed
- In 3.10, Jeremy makes a comment to Tyler about Alaric being pissed if he finds out he borrowed his crossbow, thereby establishing in canon that he knows where Alaric keeps his shit, and that Tyler knows that Jeremy knows. This was only one episode before the first murder. This could indicate a couple things.
1) Assuming that Jeremy a) was not/is not currently compelled or somehow b) didn't go to Denver for some other reason -- he could be providing the culprit the access to the weapons.
2) It was also mentioned in 3.10 that Tyler has an open invitation to the Gilberts'. He could have gotten the location of the stash from Jeremy who was off of vervain at the time. So the whole Klaus using his sire bond with Tyler to have Tyler kill off the council members, could be very true. (In which case, he can pretty much kiss his chances with Caroline goodbye after what happened to her dad.)
If scenario 1 is true, who is Jeremy helping? Katherine? But why? Why does she give a rat's ass about the council members? Especially with Klaus in town. It seems risky. Unless she has some other motivation. If Tatia is still alive, and if she is a vampire, she (or Katherine) could be compelling, or simply misleading Jeremy. (I sincerely doubt he would willingly help Katherine or anyone frame his sister or harm "innocent" council members.) This one makes the least sense, which, knowing this show, makes me most likely to believe it's probably the one to bet on.
So far, the Klaus + Tyler scenario seems like it could be the most likely and the easiest for the writers to pull off. But in my brain, it doesn't work at all. I just don't understand why Klaus would bother with winning over the council on the same night he starts killing them off? And why would he put his precious doppelganger in danger by having her framed for the crimes? Clearly her fingerprints on the weapons could be a detail he's unaware of, but then why use Alaric's weapons? It just doesn't seem logical.
I am sure there are some obvious clues that I'm overlooking, and I may have gotten some details wrong, but I hope we get more answers in the next episode or so. As much as I want to believe it's Katherine and Jeremy working together for some yet-to-be-seen nefarious purposes, I actually think it would be cooler if this storyline turned out to lead us straight to the original doppelganger.
ETA: If it's Meredith Fell I will laugh. The fact that she's pretty much their first and only suspect tells me that that's a false alarm.