Sometimes your opponent really is a morally blighted person propagating an incorrect argument to further their own self-centered objectives.
When we're talking about dealing with people online, this is another of multiple bad reasons to attack a person's character. It's a really bad idea to make yourself judge and jury on someone you have never met in person. Chances are you have made assumptions about them that are false, and attacking them is not going to draw them out of their character flaws.
Plus, we are all morally blighted on some level, so between that, your lack of knowledge about the person, and your lack of relationship with the person, you really are in no place and possess no authority to attack their character. If you do know the person personally and have a relationship with them, then perhaps there could be a time and a place for attacking their character, but it seems to me that those instances would be few and far between. In any online context, though, this doesn't work.
When you come from a place of assuming that you somehow have moral authority over another person as if you are somehow in less need of God's saving grace than anyone else, I would suggest that that is a character flaw in and of itself.
"Do nothing from selfish ambition or conceit, but in humility count others more significant than yourselves." - Philippians 2:3