I want to trash-talk someone I haven't even talked to in months
Why do you want to do that to this person?
He hurt me, but he has no idea he did that.
You were hurt by someone who doesn't even know they did it, and it's been a long time since. You should forget the whole thing and chalk it up to experience. The less you think of them the better you will feel, so just do it for yourself.
It's good to hold a grudge from time to time.
I have to disagree with you, there. Holding a grudge only causes stress to yourself, and this is always harmful to your health. Since this person didn't even know that he hurt you, he likely had no ill intentions, so you should just forget it happened and move on with your life.