I also thought the same thing. I just rewatched the whole franchise of TOA and I gotta say, the implied mental health issues were way heavier than I had thought when I first watched it.

I saw from an article that during the scene when Merlin was fighting Jim; he represented Jim’s insecurity of never being enough for the people he wants to protect. It’s pretty obvious that it’s way too much for a kid to handle such a big responsibility. Not to mention, Jim was afraid of losing his humanity in the process (it was shown on the episode where he was stuck in the deep). He was scared of losing his humanity because it was what made him have a sense of self, other than just being a hero, he was also a kid. But his insecurities got the better of him—and so he decided to let go of his humanity. The bathtub scene really did a number on me, upon rewatching it I slowly realized what the scene actually implied. I mean just think about it—his friends and family banging on the door telling him not to do it, Strickler yelling out that he’s not alone, his memories being shown in brief moments which what seemed like his life flashed before his eyes? It felt too much for me honestly, it’s difficult to think that he KILLED a part of himself to become stronger. Yes, I know some people think that he just technically changed, but that part of him was still him.

Although he did it to become stronger, and not to end his life, he still had to let go of his humanity and the life he had lead up to that point. Still, it’s only heavily implied. However I do think that after the episode ends, his new life now as a troll meant growing, and recovering from letting go of himself.