How would he ever forget that he had left Felicity on her own to die and had no clue if she survived or not?
The guilt, the pain, the bitterness and the regret, Johnathan felt them all at once. He needed to overcome those disturbing emotions somehow. So he thought to follow some traces of humanity.
He pitied Sebastian enough to show some mercy upon him.
'You were a good man, Sebastian, and you deserve respect.'
He didn't know if Alexander made it home alive or if he reached Felicity before she could die and saved her. Johnathan had committed enough sins for life, and he decided not to give up on this one single chance to earn a small concession for himself. He could do one small good deed.
Sebastian deserved a better death, and he definitely deserved a respectful funeral, a proper goodbye from his loved ones.