kids can be pretty cruel if not left unchecked
tifa is a good kid at heart, whose had to endure a bit of reflected racism, resentment and as such never got close to other people which meant she didn't learn how to see things from other people's perspective. The bandits unlike the noble's were born in poverty as such thieving, stealing, killing was just survival instinct not dissimilar to demon realm. Tifa has yet to see things for the angle of desperate people
Edit - I think people around here have forgotten what it was like to be a kid, that weird over confidence of feeling invincible until when you grow older and start finding out you have limits