Without doubt Legate Lanius.
I know you corrected yourself but it did make me smile..
Van Graffs.
They know full well what they're doing, they don't give a [censored]. That to me is evil. Imagine if the Courier met Ulysses, heard out Ulysses lesson about the consequences of his actions, then the Courier said "I don't give a [censored]" and continued that way. Yeah, that's evil.
Utter agreement..
Though Ultra Luxe is also high on my list, they aren't really evil but they are a bunch of nihilistic capatalists..
(the rate in which they can brought back to being canibals without any worry is a secondary reason)
No idea who I'd pick, there are so many characters I disagree with.
But possibly Boone.
I think he's evil because he is a sniper and he's good at it, but that's not the evil part, the evil part is how he allows himself to be used by others with no morality to it.
Follow the Courier around and kill off people? Sure.
Join up with NCR and kill people? Sure.
By that first part and using that as definition any companion would or could be labeled as evil.. with the possible exception of EDE, Lilly or Rex.
Hell, even having him join is evil, he simply doesn't give a crap, he just wants the illusion of "I killed the one responsible", not proof.
So he's a cold tool to be used for murder.
Boone kills anything without emotion if who he follows says "kill it".
He doesn't do it out of ideology, morals or ethics.
Only things he has ties to is NCR and Novac.
Also, I consider snipers to be cowards.
I think that the one you're going to kill should get a moments notice before you do it.
It's unfair for their life to just *snap* end.
So I'd say Boone is a candidate for me for a lot of reasons.
OK I'll bite as well...
He only doesn't demand proof if you can convince him.
Like I said ^by that definition all companions can be regarded as evil.
Well I understand being opposed to Legion, but he is obsessed with killing them.
In one ending he simply stops caring about the rest of the world and just guns down legionnaires.
He might have reason to hate them, but it doesn't make him any less of a bloodthirsty psychopath.
He is not a psychopath, he is a highly troubled individual who can be steered in a certain direction by the courier. You want to shape him into a psychopath .. well you can.... or you can make him care (aaaah) and let him live the rest of his life in remorse. (but you are pulling our chains aren't ya... )