With Michael it's about revenge, hence , his repeated viciousness.
With Terri's parents it about them trying to keep their helpless disabled daughter from being executed by starvation and dehydration George.
It's about saving her life. If the cold-blooded, selfish, greedy, adulterous, neglectful, miserable excuse for a husband wins the case- helpless disabled Terri will be slowly and excrutiatingly murdered.
The precceding is my opinion blah blah blah
In a sick way, Michael could be sincere and think it's "best for Terri." He's fallen under the influence of GF who behaves like a raving case of demonic possession. That kind of industrial strength brain wash is dangerous to the nth degree.