In Astonishing X-Men #29 (part of the "Ghost Box" story arc), the X-Men go to see Forge when they discover he was creating new mutants by adding a third chromosome to people's genomes. On the way, Cyclops says:
"...Forge is a man with a mutant genius for engineering who's also had one or two mental problems in the past. Even with his head on straight, he's never made the best choices in the world."
Additionally, they mention how poorly he treated Storm.
The events of the "Ghost Box" story arc do not go well for Forge:
He chooses to stay instead of escape with the X-Men as Abigail fires a 2-terawatt quantum laser into the ghost box in his base, presumably killing him.
What mental issues has he had, and do they justify his actions here?