I want to be petty towards my ex

I want to create a fake profile and Comment FBI OPEN UP on the picture he posted of his new (ex from couple years ago but back together) girlfriend. Why? Because she was 13 when he was 18. He just turned 22 and idk how old she is but still in HS. I really want to, but know I shouldn't and won't. And this is not the only underage girl he's been with. I was the first girl who was older than him that he's dated. Ugh glad to be done with him and won't do it but ugh the urge is there