Attacking someone's credibility has no bearing on that person's argument. A true statement spoken by the devil doesn't make that statement false.
Next, it's very hard for someone to correctly divine the motives of another. It is often the case that it's easier to make up base motives and assign then to an opponent than it is to refute their argument.
Finally, that people "decide their position on an issue and then seek supporting evidence" simply shows that "people" are idiots.