With what to gain?
As you said, he had his loot, he had his screwpal -- her parents offered to care for her, bada bing, bada boom, he's gone.
But he didn't do that. It couldn't have been because he was after Terr's trust fund (unlike her parents) -- in 1998 he offered, in writing, to donate that to charity.
So, Mr. Non-Dense one, why did he stay? Why did he want Terri to die? You're so friggin' smart, tell me.
That is a good question, why? The only noble reason would be it was her dying wish - but he only came up with that after the money. Why not let her parents care for her and move on? Why you ask? He wanted her gone obviously. Why do a few husbands kill their wives?
As long as they were still married, he was her heir and has rights to her story.