It's actually quite well written. I wouldn't reply or engage with them, they're obviously disturbed. It sounds like maybe written by somebody who has personal experience. And is bitter/depressed and maybe has done transition hard. In their mind they might believe that's what transition is, and might think they are somehow trying to help you avoid pain they've experienced. Or they might just be expressing their own bitterness in a kind of cry for help, wanting attention of any sort. You prob need to work on your DGAF armour though and keep yourself safe from caring too much about what others think or say. Before I transitioned I made myself read all the trans regret stories, and lots of comments like the one you got. I wanted to go into my transition with my eyes open, to all sides of the story. This message is one side of the story and for some people, it is how transition has been for them.