Social Darwinism and Morals

[lucksack]

Here in Finland we had this guy that went to shoot 8 people in the name of natural selection, and then himself. It went like this:

He decided he had abandoned slave morality and created himself a master morality which is based on natural selection. He also described this as "going through synthesis and becoming an overman". (He also said he had "refined" some of Nietzsche's ideas, so he seems to define an overman as someone who creates his own morality and thinks about philosophy and existentialism.)

Then he decides he wants either all humans destroyed or human race to evolve to a better direction, and he wants to help in that so he makes this terrorist attack to kill some people and also to get a ton of publicity for his thoughts (and himself).

However, when he went to do the shooting, it seems that he started to feel pity for the victims, he shot a few first and then started to let people live, he also didn't shoot nearly all his bullets.

Now it seems to me that the big logical problem here is, that he only thought he had achieved this "master morality", when truely he had not and was really thinking like he used to deep inside. Is this a valid argument? Other good arguments, that might make a similar "social darwinist" change his mind, if one is planning a massacre? (And no, I'm not planning one, and neither is anyone I know.)