"It wouldn't help anyone." Staying would have done nothing- for himself or for resolving any aspect of what happened. Full penance can be made through action, rather than doing nothing. For Kal-El, at least, it least had some form of help for him in a way, even if it could not last.

Besides, it would be too much time to deliberate and think on his own actions, not only what happened in Kithika, but since his arrival here in the first place. His original death. It was something he tried to avoid thinking on in depth. There was only so much flexibility in thought that even he could manage.

"He's aware of that. It hasn't changed his decision." Kal-El is an adult, and entitled to his own choices.