At least she doesn't even have to think about whether to fight this guy or not. He's hurting Steph, and that's enough reason for Minako to jump into the fray without caring about this person's wellbeing.

He can't be a good guy if he's going around beating up random girls.

"If you wanted to talk, you shouldn't have started fighting instead!"

It's all Minako says before she runs at the person, making sure to zigzag as she goes too in case he might shoot again, and attempts to swing her sword at him when she gets close enough.