Ehh this is probably the most dishonorable thing he does in the game
- Spoiler:
There's two things they could be referring to:
1. Shooting the hostage and the Fetus. While it is a d*ck move, Vergil has never really been one to honor agreements as this cutscene shows. Really, the only times Vergil behaves honorably are during his fights against Dante.
2. The Assault rifle. Vergil has made it known that he dislikes using guns because they are not weapons fit for a "true warrior". Despite that, he does use magic to summon what are essentially homing soul masses in the shapes of swords. And while he does not like using guns, he has been shown willing to use them
As for why he's using it? he's pretending to be a human so the big bad won't know that there's someone else like Dante around.