This is absolutely abuse. The fact that he would get physical with you because you forgot the light alone is abuse. The idea that he would threaten suicide because of the fight says he is an extremely unstable person. Then he threatens divorce and takes it back but at the same time goes to the basement to live?

I know it's hard to but you described things as if things are relatively good with you two and then you describe an extremely unstable, unhappy and abusive man and a very extreme episode. Think about the relationship. What is going on taht you haven't told us? I ask that not assuming you've kept it from the post (only you know that) but because maybe you've been in denial about something and have not seen the signs? Thing is, it would be rare to see this extreme response to something as small as a light being left off all off a sudden after 3.5 years. I know it could happen but I'm sure it's rare. Something is missing from this description. Either he is in the midst of a huge emotional breakdown (if this is brand new behavior) or this has been going on and has just now escalated. Either case, you don't only need marriage counseling, he needs help. Maybe even more than the marriage counseling at this point. Something is very very wrong here.

A simple solution for this could have been that he turn on the farking light when he leaves! wait what? he didn't even think of that?

I agree with Leed. he wants to virtually separate from you by living in the basement? Then he doesn't get the benefits of calling on you when he wants sex. You're not his toy, you're not his wh*re to be called on to meet his physical needs only. He wants that, he should have his divorce and go live near a Cat house.

If he's unwilling to get help, either with you but especially for himself. Get out. I'll make no bones about this. He won't acknowledge his abuse and get help, it's not going to get better but worse. he probably only lives down there because he couldn't afford it and his spending habits at the same time.