An essay here, because I am seriously paranoid about how I play Vergil, and will not rest until I just get some things off of my chest, so you have a little more understanding of my portrayal of Vergil. No, I haven't covered everything about Vergil in this post, I just got straight to the point I was trying to make about Vergil's "odd" behaviour here (and not getting into the whole of his mentality / beliefs), so you know I have my reasons to how I am playing him.
I believe that Vergil is very human, he loves Dante as much as Dante loves him, but is also a complete lunatic (and after everything he went through and his demonic side conflicting with that, do you blame him?). IS THE EMOTIONALLY WEAKER TWIN (and later revealed the physically weaker one, as well), and is actually mentally unstable. See, to me, there's a lot more to Vergil than a simple "evil", because he does have a heart which stops him from getting anywhere with his plans in the end, as it throws him out of focus. It's both of his human and demonic side that drives him, but Vergil has very little understanding of this and prided himself in being demon he was ignorant of all else.
He never says he cares or loves, he prefers to show it, and he is capable. He just believes actions are stronger than words. Their demon side enjoys a good battle while their human side is cursed with so many painful emotions he cannot stand.
He plans to attain his father's power, he so much as tells Arkham that the true use of Temen-Ni-Gru and all that would be resurrected was of no interest to him. He just wants that power. He is prepared to unleash Hell on Earth to get it. After Dante & Vergil's first fight, Vergil stabs him one more time, to awaken his Devil Trigger. While he does taunt Dante, he is also teaching him ways to protect himself and fight (perhaps when Hell is unleashed). There was no other reason for Vergil to stab Dante with Rebellion, as he knew, from experience, their Devil Arm activates their powers. This is quite an insane way of showing one that he cares.
Vergil does come across as rather insane, though he may appear to be normal, there are certain actions he makes and things that he says that suggests otherwise. What caused him to be like this was the death of Eva and the total misunderstanding of his father’s legend. He was a son of a hero - a warrior. Of course, he shares some of that heroic blood and was under the impression Eva was killed because he was too weak to prevent it, and if Sparda was there, or someone with powers to match, it would have been prevented. Over the years, this would have become rather frustrating for him, driving him mad in the end.
It was clear he wanted to protect another as a child, at least, because during the demon attack, Vergil actually attempted to fend off the demons after making sure his brother was safe. Being a weakling and only a child, he was easily and quickly over-powered (this moment being where he was stabbed with Yamato and his demonic powers activated), he watched as their home was burnt down, and all he could do was uselessly cry out his brother’s name. In the manga, a demon (Arkham in disguise, actually) approached Dante, telling him that if he was strong enough at the time, his mother’s death could have been prevented - Dante did not listen. Perhaps Vergil was poisoned in a similar way, while his head was booming with a maelstrom of emotions, regrets, responsibility and frustrations, was quick to agree, desperate for power. Follow the dark path, gain more power and your loved ones are protected (to be honest, a lot like Anakin Skywalker. Whenever I watch Star Wars, I lol. So hard at the similarities.).
For example, he states he wants power to protect another -- he never realises that it is his human side that yearns to protect another. But it's his demonic side that drives his mere lust for more power and wants it for the sake of becoming like the almighty Sparda. Both he and his twin blame Sparda for their mother's death, but also themselves as it's their mother's death that drives them down that path they walk.
See, when Vergil was a child and snatched away from light, he would have had very little understanding of his father's legend and tormented by the death he could have prevented. His head would have been filled with two voices: a human and a demon one, wanting to satisfy both. And perhaps, that way, he'll hurt no more and regain some form of control.
MORE SIGNS OF BEING INSANE IS AT THE VERY END OF THE GAME. He throws a tantrum and makes a great big fuss over Dante's words and refusal to cooperate, with Dante threatening to kill Vergil. Vergil just briefly and vaguely states he needs more power. They fight, Vergil loses, letting go of the power he desired and the amulet shattering from around his neck. HE WADDLES OFF TO HIS MOTHER'S AMULET -- DOESN'T WANT POWER ANYMORE, AND HAS LEARNT A LESSON - IT WAS THE ONLY THING THAT TRULY BELONGED TO HIM (he chooses the amulet over power he wanted so badly). AND AFTER BABBLING ABOUT STAYING WHERE HIS FATHER LIVED, HE JUST STROLLS OVER TO THE EDGE OF THE CLIFF AND THROWS HIMSELF OFF OF IT, SMILING. Like all of that was normal behaviour and nothing happened. Take note of his tone of voice, he sounds rather cheerful, sure of himself and he actually looks as if he's not all there.
To me, he is not all that evil, but a tormented soul. He was violently ripped away from comfort of family and raised by demons. His lust for more power was only encouraged.
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So, for a little understanding to the recent plot where he had stabbed his mother, you can understand why it has been such a major blow to him, as well as quite distressing and painful. Power corrupted him, and blotted out all that was good, but was under the impression he was doing the right thing, showing that he cares. For him to stab his mother goes back on everything and he ends up being no better than the demons who claimed her life in the first place. Add to the fact that he suffered from the Mental Regression Virus, reminding him of the real reason he started on his journey to attain more power.
So, if you have wondered why my Vergil might act differently to how you might have expected, you know my interpretation on a few things.