I don't know why you suppose personal attacks are the prerogative of theists, beso. Looking back through this thread, I see some personal criticism, for instance, of Mallam, who dared to disagree with the originator of another thread. She claims he ruined the argument; it seemed to me he just won it. Whatever, it hardly seems to me that his alleged personal wickedness had any effect on the substance of his argument. So why raise it?
My own answer, for what it's worth, is that personal criticism is endemic to humans, regardless of whether they're believers or not, and does not 'serve' one side of the argument better than the other.