Pretty sure Rokudou was trying not to fight because he wants to be a good example for Ranna, not because he wouldn't be justified kicking this dude's ass. It's hard to teach someone not to resolve their problems with violence when they see you fighting. It just backfired, because he underestimated how dumb this dude was. I'm not even sure you could call it a "fight", the dude only hit him twice before Ranna finished him.