Welp,,, i think given from the last scene theu changed themselves and accept that fact that they can't do anything about the past, but to look forward for the future of both of them...
Their past relationship were indeed toxic but i think their current isn't one. Well this was in my opinion only
dohyun has stockholm syndrome the fact that dohyun was ok with living with his rapist and kidnapper proves that and seungho clearly acknowledged himself that he is dangerous and that there's still a possibility of him hurting dohyun that's why he was pushing dohyun away in the first place the ending is a perfect example of a false happiness their relationship is fragile one of them may end up dead the next time they have fight or misunderstanding
i honestly have no idea why you think that their current relationship isn't toxic
do you really think it's healthy to fall in love and live together with someone who kidnapped you and raped you?
Definition from urban dictionary to describe toxic: a person who is tainted by a subconscious malevolence or psychosis that affects the life of those who come into contact with them.
you should probably research more
yall seek a therapist now if you think their relationship isn't toxic