The rule of law at the time was the spouse was 100% responsible for making all decisions.
It was exactly like Adolph Hitler used to do. Bring them in the front door and they never come out alive.
A guardian who is an adulterer is supposed to be terminated. That's just one violation of the law. There are pages of violations leading up to Terri's murder.
Michael's authority rested solely upon his legal guardianship. Although spouses have legal priority when seeking guardianship, they are required to meet certain requirements and abide by certain conditions to maintain guardianship. If a guardian fails to meet the requirements or acts contrary to the necessary conditions, the guardianship is to be voided and a new gaurdian assigned.
Michael's violations of the guardianship requirements are too numerous for me to list from memory. His openly-stated intention to marry a woman by whom he fathered to children, however, should have been sufficient to void his guardianship as soon as he tried to have his wife killed. Although it is legal for people to be guardians in the presence of potential conflicts of intrest, guardianship is to be terminated if guardians put their own interests ahead of their wards'. Michael had a clear craven personal interest in his wife's death. His claim that his actions were motivated by a love for Terri rather than his own obvious craven interests would be laughably absurd were not Judge Greer unwilling to hear any challenge to it.
Indeed, it's remarkable how Judge Greer was always too busy to hear challenges to Michael's guardianship in timely fashion, and never seemed to object when Michael didn't even bother to show up at such hearings when they finally came around, and yet he somehow always seemed to find time to hear Michael's motions to kill Terri.
Of course, ideally one would vote Judge Greer out of office, but that's somewhat difficult when state laws make it essentially impossible for any competitors to campaign (the state laws would also seem to have forbid Judge Greer from campaigning, except that he openly flouted them and nothing happened). It's rather hard to unseat an incumbent when he's allowed to campaign and his competitors aren't.