Good and evil is kind of relative here.
Lady kills her own father.
Vergil almost kills his own brother, is happy to kill Arkham at the door of the Lair of Judgement, and helps raise a tower that gets a lot of townspeople killed.
Dante is willing to kill his own brother to stop him having more power, and more than happy to kill Arkham.
Arkham is willing to kill just about anyone to get what he wants.
So they're all willing to murder someone else to get what they think is right. The only difference is what the belief is. Whether you kill someone to save other people, or for your own purposes, you're still doing it because of personal belief.
Vergil isn't evil, he's selfish. At no point does he directly kill anybody who doesn't rightly deserve it, he only kills those who are a threat to himself or are putting themselves in the way of his plan. He is not a sadist, or revelling in 'evil' as Arkham is. He indirectly kills people with the raising tower, but he probably considers that collateral damage for the greater good. Dante considers Vergil's death collateral damage for the greater good, and Lady her father's death, etc.