I wouldn’t mind jmart being unhealthy at all if their relationship hadn’t been so… ordinary.
Let me start off by saying that it isn’t an inherently bad thing to portray toxic relationships in media. Moreover, the toxic relationships do not necessarily need to be painted as wrong or punished by the narrative. More often than not, unhealthy bonds are far more appealing to the audience than healthy ones.
I would be more forgiving of jmart being toxic if it wasn’t so very boring. I said before that making Jon and Martin a couple was essentially putting two coworkers together and making them walk through the apocalypse. They don’t know each other outside of the Institute at all.
Everything Martin knows about Jon is that he likes the tape recorders, and for most of the time he believed Jon to be a good man. He developed a crush on the man presumably because his rudeness and callousness towards him reminded him of his mother, so it was familiar.
Jon didn’t even know that Martin made up his last name. All that he knows of the man he learned during his period of paranoia-induced interrogations and stalking. He could, of course, learn infinitely more about him now that he became an Avatar, but considering Martin’s repulsion to the Eye and the Fears in general, I doubt that would improve their relationship much. If anything, it would push them further apart.
Even the toxicity of their relationship is so basic. The fact that they are in a world ruled by ancient fear entities didn’t add any interesting twists to the nature of their bond.
Martin belittles Jon for his monstrous urges and abilities, only being tolerant of them if it benefits him. While the killing spree was not explicitly started by Martin, he was the one to urge Jon to continue despite the latter’s hesitation and shame. He ordered Jon to kill the avatars that he personally disliked (Simon, Oliver) but he suddenly wasn’t all that murder-positive when it came to the ones he did not know personally (Jude Perry). He made an attempt to apologize for it, but he took back his apology almost immediately.
Jon takes it all quietly and without complaint because he hates his new self as well. It’s not at all a new thing, not even for him - Basira and Melanie already hated him for being an avatar waaaay back in season 4.
An interesting bond requires a foundation. Jon and Martin do not have that. They did not have enough time to develop that. Almost immediately after they got together the Eyepocalypse started, and that is not a good environment for a blossoming relationship.
They both need eachother, they need closure, they need someone, anyone to latch onto. But they don’t even know eachother as people.
Even their mutual desire to sacrifice themself for the other feels selfish. They wouldn’t be able to survive on their own. Without the other, it’s likely that they would’ve succumbed to the Fears fully, and neither of them wanted that.
It’s easy to offer to die for the other. It’s infinitely harder to let go.