Would it be all right if I troll this one guy on the internet?
Why would you troll a man on the internet?
I don't agree with his stance on gun laws.
It is okay to disagree with people, however, trolling them on the internet when you don't like what they have to say is unacceptable. Malicious trolling hurts people's feelings.
Nah, it's not mean. He's just an asshole anyway.
If this man is being an asshole, then that doesn't give you the right to sink to his level.  You can try to change his mind about issues, if you have salient evidence, but if he just keeps being mean to others, then you should call him out on that.  Making him look foolish for his own behavior is more effective than being a fool yourself.