You are a 'soldier' you were trained from a very young age. And they forced you to kill innocent people as part of your training, the ProHeros were looking for those 'soldiers' to know them, thanks to Hawks they found the base.

They interrupted, 'attacked' your mates but you shot some heroes, Bakugo (a UA student) attacked you from behind. When you wake up you was in a dark room.

You didn't know where you were with nothing(No weapons) Aizawa entered.

Aizawa "What's your name?" he says coldly to you