There are people who do not like the idea of murdering babies and selling their parts who are not Christian. Understanding that something is wrong doesn't take a belief in a deity. In fact these people don't have the constraints put on them that Christianity puts on its followers so a violent response is more likely to occur.
I won't go so far as to speculate whether Scott Roeder or Eric Rudolph were literally atheists, but they had rejected Christian teachings as much or more than any atheist. Both held/hold Christians who are Pro-Life in utter contempt, as enabling abortion by being non-violent.
These are facts that Pro-Abortion true-believers deny; DU seems to have more than its share of such. To be honest, I don't mind seeing a Pro-Abortion guy like d3600 getting embroiled with the Pro-Abort true-believers. He might learn somewhat of Pro-Abortion fruit and be repelled thereby. Sometime being unwillingly awakened to the horror that is abortion changes minds and hearts.