He always had the option of moving on, simply by divorcing Terri. The family begged him to. There was something more evil here than him being "fed up" or "ending his own agony." He had been cruel to Terri and hateful to the Schindlers over the years. You see the same evil when you say, "he continually, even after her death, stuck a shiv in the backs of the Schindler family, happy to inflict whatever pain he could." Michael was -- beyond a reasonable doubt -- moved by hatred when he killed Terri.
Money was also a motive. He wouldn't divorce her because her estate might be worth millions in book and movie and TV rights. Michael has always been a hound for easy money and if it's dishonest or if it profits from his wife's blood, why, so much the better.
Let's not forget that while she was being killed, Schaivo was offered $2 million by an outside party if he would just "walk away."
When he rejected this offer, the culture of death praised him and their media allies gleefully informed the world, "see he really does love her, it's not about the money to him."
The reality (at least in my opinion) was two-fold:
1. He NEEDED her to die because if she recovered, she would be able to tell the world that HE WAS THE REASON she was there in the first place.
2. He calculated that killing her and thereby becoming the sole heir to her estate and his book about killing her would net him far more than $2 million.