What we do know is Mike was alone. After Mary's death he probably shut down completely. It looks like he even went blind.
Blindness, through retinal damage, can be a consequence of diabetes. But it is easily avoided through medical attention and is almost unheard of in countries where doctors are available.
If Michael is blind, that's a clue that he may have shut down to the point that he stopped treating his diabetes. Untreated diabetes also leads to cardiovascular disease, which was probably a major factor in his death.
Perhaps the end of Michael was self-inflicted, over a period of years.
