Why did she have to be put down?
What predicament was she in? She wasn't terminal. Her family was willing to take care of her.
Why did Michael have to put her down?
Legally whacked for all the world to witness one of the most barbaric acts evah! Starving and dehydrating a woman to death.
Because it's horrific to keep someone in that condition when there's no hope of revival. Most people would not want to be stuck in a body like she was, with no cognitive brain activity and no chance of recovery, so it's not farfetched to believe that she conveyed as much to her husband prior to her unfortunate demise.
Even if she didn't, as her husband, it was his call to make.
There was nothing for him to gain from her death. If he didn't do it out of love and compassion, he could have simply divorced her and let her parents drag her horrible situation out. Clearly, he did it for his wife.