Why? Unless he's arguing on the basis of authority ("I say so!"), reasoning is reasoning, regardless of whose mouth it comes from. If his reasoning holds up, what basis is there for giving it less "weight"?
If you think the arguer is biased against the position being argued for, that's maybe grounds for suspicion that the reasoning is flawed in some (perhaps subtle) way—but suspicion when listening to arguments is always a good thing.
Anyway, as has been pointed out elsewhere in the comments, his position isn't actually inconsistent: it's perfectly rational for him to think that a system where all parties are equally prohibited from certain abusive actions is better for him than the system where they aren't, even if he's participated in the abuse in the past. He may very well have come to the conclusion that he loses more from the abuses of others than he gains from abuses of his own. [The "hockey-helmet" example someone gave is a good illustration, I think.]
If you think the arguer is biased against the position being argued for, that's maybe grounds for suspicion that the reasoning is flawed in some (perhaps subtle) way—but suspicion when listening to arguments is always a good thing.
Anyway, as has been pointed out elsewhere in the comments, his position isn't actually inconsistent: it's perfectly rational for him to think that a system where all parties are equally prohibited from certain abusive actions is better for him than the system where they aren't, even if he's participated in the abuse in the past. He may very well have come to the conclusion that he loses more from the abuses of others than he gains from abuses of his own. [The "hockey-helmet" example someone gave is a good illustration, I think.]