do you think that undyne is on side of the justice?

yes, undyne actually battles you because she was ordered to by ASGORE, but you save her becoming closer and closer friends she figures out, with everyone, that you shouldn't take your soul. she can also figure out that you are a force to be reckoned with and almost DIE for the dino kid just to reform with the fact that she can "feel everyone's heart beating as one to kill you!" when defeated she tells everyone to evacuate so they don't get completely murdalated.

Yes. Even if she does attempt to murder you, it's most likely because the Underground hasn't really seen too many humans, therefore they can only go off of what they do know. That fact being that humans are horrible people.

Yesss!!!