This could be seen as more of a "medical feasability" question, but I was considering this possible scenario that would only add to Michael's ultimate come-uppance from the end of III to his death.

Could it be that Mary's death was so painful to Michael that his silent scream and yelling brought about another diabetic attack that left him permanently blind? It seems as if he is blind in his death scene. If he did indeed go blind immediately following his daughter's death, it would mean that, literally, the last thing he saw was the death of his daughter.

Chilling, cold, and ruthless - just as Michael was at the peak of his reign of terror.