Thunder just remembers his past. The time when Magneto was alive, they were safe, Thunder wasn't so evil.
Magneto's age on the pic: 14
Thunder's age on the pic: 5
Well, they escaped from the scietists, and when they we're close to da freeeeeeedum, Magneto was shot in the chest. Thunder went insane, he killed everyone who was in his way. And that day when he was insane made a really bad appearance on his psychical side. He just stopped to trust humans. If you wanna know more, then read "Experiments of pain". It's unfinished, but... At least something..^^