We know that but Bobby doesn't know that. And let's be real here, Bobby is probably the last X-Man to do something so brutal but he was in painful mourning. I don't like the extreme violence either but I understand what happens when all you see is red. This is the same group allied with someone shouting "Death To Gene Junk! For Verendi!" while throwing a grenade. Had Bishop not intercepted it, the ship would have sunk.