Character in question: Mid-level military officer who was ordered to bomb two friendly (and alien, with their own races, not colonies of humans) planets whose loyalties were sort of in question with very, very large thermonuclear warheads to take out certain enemy positions. The planets weren't exactly loyal to the central government and the enemy (and large portions of cities) were destroyed to make a statement. He did this in a fighter without support from any other craft, so he was in essence a lone wolf, but acting under orders of the highest military authority. Immediate death toll: north of 250 million (I haven't addressed the greater problems of post-apocalyptic issues and I'm not planning on going there).
Have googled: Catholicism, confession, mortal sin, venial sin.
The problem/question: His soon-to-be wife wants him to go back to confession. What category does this sin fall under? I mean, it would seem to me like it's a ticket straight to hell in the express lane. Would this even be forgivable, even though it was done as ordered by a military (and refusing probably would have resulted in a lot of friendly losses, including a major attack on Earth itself)? It's not like this has ever really happened.