I don't think Kurono was being out of character when he left the room. He never asked to be there nor did he exactly enjoy seeing people die or hunting aliens. The only reason why he put up with Gantz after the Chibi mission was because of Tae. Why would he stay in that room anyways when the purpose of his motivation to do well was to revive Tae and leave?
Kurono is not really obligated to saving lives or revive the deceased. Kishimoto is not very useful compared to Reika in missions. Bringing back Sei would ultimately destroy his relationship with Tae. He was practically peer pressured into leaving, which ironically got him killed.
etufo: HS is most likely going to die sooner or later. Who will kill the cool character killer?
Popo: well, obviously going by that logic, HS will have to kill himself...