I would suggest this guy worked by his own motivation. It's sick and it's tragic and goes against anything any reasonable human being stands for, atheist, Christian, Muslim, Jew or otherwise. When we talk about 'one true Scotsman', we are tending to talk about the doctrine of a religion and things it teaches people but atheism has no doctrine, there is only one single requirement - lack of belief in any deities. I mean a religious person can be motivated to do wrong outside of their religious doctrine, without trying to invoke Godwin's law, but I think it is genuinely relevant, Hitler, he believed in God and was a theist and followed the Christian doctrine, but his motivation was unrelated. I think when talking about murder in relation to religion and irreligion it is that distinction we have to determine. I don't think atheism or theism alone could be motivation for murder, theistic doctrines like the bible or the Koran on the other hand are capable of doing so. It is possible there are atheistic doctrines or philosophies out there that could do the same, but I am unaware of any. The most popular atheistic philosophy seems to be secular humanism and that says nothing that would be motivation for murder.
One true Scotsman comes about because we can't determine what a 'true Scotsman' is, the Christian doctrine (for example) is actually pretty ambiguous - are you meant to murder in the name of God or are you suppose to be completely peaceful and forgiving? Different Christians argue differently and to know the 'true' meaning of the bible we need a magic decoder ring. But other doctrines or philosophies out there aren't necessarily as ambiguous, sure there may be variations, for example, in Utilitarianism, Jeremy Bentham and John Stuart Mill had varying ideas, but these variations aren't direct contradictions, John Stuart Mill was trying to improved on Bentham's ideas and of course there is a degree of individuals shaping the philosophies with how they see things, but it's a lot easier to determine what a 'true' Utilitarian is as its criteria isn't all that ambiguous. The main principle we're talking about is 'the greatest benefit for the greatest number', so if you met a Utilitarian who sought to exploit a number of people for personal gain, you could say they're not a true 'Utilitarian'. I assume we could apply the same logic to secular humanism. Of course, a person who bases their morality on Bentham's philosophy would not be able to call somebody who follows Mill a 'false Utilitarian'.
When it comes to a religion like Christianity, how do you determine what the bible actually intends? We have so many contradicting principles that these variations are a direct contradiction. We have "thou shalt not kill" and "kill homosexuals and those who aren't virgins on their wedding night". We have, "I, Jesus, bring peace and forgiveness" and "I, Jesus, who's come to enforce the old law." We've got a God with a multiple personality disorder.
However, getting back on track, can we be sure that he was an atheist? Hating God doesn't necessarily mean atheism - I think people can hate the concept of God and it's possible that's what was meant, but I think we have to be certain of that as a person can have that dilemma but still have an active belief in God. Another factor to consider is there's different types of theistic, though I don't know much about theistic Satanism, I really only know details about LaVeyan Satanism and that's atheistic (where Satan is a symbol of individuality and not an actual deity).