He was also a believer in the supernatural. His profile on the conservative site Gab included the statement“jews are the children of satan.” Atheists don’t believe in Satan.
Furthermore, no non-theistic organization is celebrating the 11 deaths. No atheist group has a problem with people peacefully worshiping or conducting religious ceremonies. No late-night comedian (with a lone exception) openly condemns religion; rather, they point out religious hypocrisy and they’re right to do so. (Stephen Colbert, who’s been relentless against Trump, is famously and devoutly Catholic.)
Hell, Trump has likely done more to turn people away from religion — certainly evangelical Christianity — than any atheist in the country.