"Why should I care about the Jedi?" he spat, the Force around him gathering like thunder clouds. "They were weak. Compassion? Compassion is weak!"

Look what compassion had brought him. He should never have left her alone on Starkiller, should have just brought her to Snoke as soon as he realized she had the Force. What did he have to show for it? A dead master and a bond to someone who betrayed him.