Absolutely not. But if person, without leaving any letter as to motive, chooses to go into a church and slaughter worshipers, and it turns out later that, wow, half his Facebook likes were of atheist pages, well, that provides reasonable basis for ascribing a motive: "This guy was an atheist who didn't merely not believe in God, but had animus against believers, to the point that he decided to murder as many of them as he could." It's pretty obvious. Some atheists are just taking an intellectual position. Others actively hate religious people and want them expunged. It's the latter that are a problem. It's very likely Devin Kelley was one of them.
