Is it really, "pure evil", when he had a very good reason to do so?
Yeah, I honestly think it is. The path to hell is paved with good intentions. You have to know when to stop, I would think. Drawing a line could make all the difference.
Edit:
See, here's the thing: Dante and Vergil have both gotten along and loved each other when they were young. They most likely grew to love each other after they were reintroduced to one another when Vergil asked that they meet again.
Severing a connection like that so extremely could very well be considered "Evil."
"Depraved."
Given into complete Madness.
Dante could've always just imprisoned Vergil, but he chose to go for the kill, instead, releasing the demon within and making his hair permanently white.
It was only through Kat's intervention that he managed to stop and control the demon at the very last second before giving into complete "Depravity."
However, by that time it was "too late." Dante had already made the
physical (but not "Spiritual") transformation because you can see it in his hair and his mannerisms afterwards.
"I don't know what I am anymore."
He is Lost.
A Lost Soul. All because he had to go for the extreme and
try to kill his brother permanently (attempted murder still makes a person demonic, so imagine what it had done to Dante after trying to do so with his own
brother).
... Until the sequel, anyway.
--------------------------------------------------------------------------------
Edit:
However, in the heat of the moment, one could see how Dante had thought that Vergil would be capable of escaping whatever prison Dante put him into (since Dante himself had managed to escape, as well).
So, as cliche as this sounds (my previous posts have been mostly cliche, I know) killing was a
necessary evil.
Dante had to put his himself into the filth of his own potential insanity and come out of it without losing his own humanity to it in the process.
A formidable feat, any way you try to slice it.
-----------------------------------------------------------------------------------
In my humble opinion, he could've very well lost control and went after Kat, as well; especially since she was yelled at (aggressively) by Vergil for interrupting him, even after all their years of (romantic?) friendship.
Dante looked at her with... I don't even know what when Kat asked that he stop. He looked as if he might've lost control there.
But he didn't. Dante performed the Necessary Evil and sacrificed his own Angelic traits for the freedom of humanity.
Not for the "good" of humanity like Vergil said, but the the "freedom" of humanity.
Dante knew that he was better off letting the humans take care of themselves, and that he was better off killing (or attempting to kill) his brother because he knew he had to show his brother (who was the ringleader up until that point) that he was
serious, and that he had to make sure Vergil knew that Dante would stop at nothing to ensure humanity's free rights.