"How do they know a Christian wrote it?"
Perhaps they clicked on the bio of the person who posted it? SweetLiberty identifies herself as a Christian in the first sentence of her bio. I believe it's her comment that they're quoting.
Here is the original quote.
I am starting to feel like these lowlife scum should just be mowed down like terrorists. In a sense they are....cultural terrrorists, who won't rest until they contaminate and corrupt everything sacred to us.
Here is the author misquote from this article.
These lowlife scum should just be mowed down like terrorists, writes one good Christian."