Anyway, Vergil is honorable in some senses. He wouldn't cut Dante down if he weren't at full power, unless they had just battled each other and Dante was hurt badly, obviously. Then, as Nero Angelo, he showed honor, as Dante explicitly remarked.
Though I don't know about him killing Arkham in the beginning to middle part of DMC3. He may have thought honor didn't apply in that case, as Arkham had become demonic, and not in a 'good demon' sort of way.
As for people possibly dying due to him bringing back the Temen-Ni-Gru, I'm not sure that has anything to do with honor. He wouldn't kill everybody if he needs to - at least, I haven't seen anything that suggests that. It seems like everybody who lived around that area got away in time.