The problem with him being a chud is that it makes no sense. It doesn't fit any part of reality as I know it.
Ex-Muslims who realize that "Wahhabism is the true Islam" do not go and commit terrorist attacks in their style. Right Wingers would not target a christmas market. Atheists do not shout Allahu Akbar.
How does any of that make sense?
If he really was an atheist, the only explanation that makes sense to me is that, shortly before he committed this act, he came back to Islam. Ex-muslims are hated among Muslims for many different reasons, but there are those who make it their lives' mission to reconvert as many people back to Islam as possible.
I can see someone getting through to him and convincing him that he was misled and atheism is evil ("look around this country, brother, all this degeneracy is from atheism"), which made him believe he made a terrible mistake of leaving Islam.
But how could he ever make it up to Allah? Well, by driving a truck into a Christmas Market.