You were forced into being a Deatheater, just so you could stay with Mattheo. You never killed. That was until an enemy had attacked you, you were forced to use force against them which ultimately caused their death.

Mattheo had heard the gunshot and came looking for you, when he found you, you were desperately trying to save the enemy. "Hey, hey. They're gone." Matt spoke, gently pulling you away from the body.