Snopes does lean left, but they do check stuff out and confirm it before posting it. Have they made errors in the past? I wouldn't doubt it. But I think overall they are quite reliable.
As far as the shooter being an atheist, I've been hearing that today on Christian radio. Even if it turns out to be true, I condemn him as strongly as I condemn anyone else who commits horrible acts like this. But despite what conservative radio hosts like Matt Barber and Janet Mefferd want to spout, being an atheist doesn't make someone any more likely to do something like this, nor does it mean that atheists "don't want to be accountable for their actions" and feel they can go around hurting people just because they don't believe in God. Nor does it mean that being an atheist means you hate God, hate Christians or hate Jesus. I trust you already know all of this.