"I never meant to hurt anybody" Nathan knew what he was doing hurt people. He knew what he and Jefferson were doing was wrong. The call wasn't begging for help or asking anything but to warn Max (even if it was too late) about Jefferson. The fact that he knows Max needs to be warned proves he is aware of his situation. The guy has mental problems but he belongs in jail one with psych services but still a jail.

People say lots of things when they're lying. Nathan knew he was hurting people, no doubt. That doesn't mean he has the capacity to feel remorse- even if he has the capacity to claim he feels remorse.

But- even if he did, moments of lucidity do not make someone sane. Take for example King George III of the UK. He's known as "The Mad King" for a reason- he had periods that were months long where he was a danger to himself and others and spent much of that time locked in small rooms, because it was the 18th century. Being a monarch his letters survived, and during one of the most intense periods of insanity in his life he wrote a handwritten letter to his wife saying words to the effect of "I know what kind of strain my condition puts on you and the children, and I am truly sorry for the pain my condition is causing to our family". That is, in the middle of one of his weeks long episodes he had a moment where he was lucid enough to understand what his actions had done to other people and to feel remorse for it. Mental health challenges are not as black and white as you're suggesting.