I'll admit, that's actually a half decent argument. Until you look at the evidence.
He didn't shoot Rosenbaum until the very last second when he was literally less than 5 feet away from catching him https://www.youtube.com/watch?v=b9sGEbDry64
When he shot Anthony Huber, Kyle was on the ground, had just been jump kicked in the head, and Anthony was toppling onto him.
Gaige admitted IN COURT and UNDER OATH that Kyle did not fire until Gaige had raised his gun and advanced on Kyle.
So, is that clear enough for you?
They followed Kyle and they made the move to go and attack him. That's not self defense.
Good for him, maybe he should've thought twice before trying to kill Kyle.
Where was the national guard to keep people safe in crisis?