When mine went back to rehab the second time, I made it clear that we would be separating. (He was "getting sober" so he could visit his parents.)
I remember him also making the suicide comment... except his was more like "I have no way out... I might as well..." We were lying in bed, I was exhausted after a long day at work, and I was pretty fed up with the BS by then. And annoyed that he was keeping me awake. Even though you aren't supposed to do it, I called him out on it, in a "how dare you use a suicide threat to manipulate me" sort of way. He never mentioned it again.
I too felt that cold calmness that LexiCat described - and I packed up as much of his stuff as I could and shipped it to his parents along with his car. When he said he wanted to come back for a "couple of days" I told him that there was nothing here for him, and booked him a plane ticket directly to their state.
I am rebuilding, divorcing, and will be paying alimony for the foreseeable future. But I have my peace and my sanity, and it is SO worth it.
Be strong.
D