As the movies progressed, it seemed that Tom was the only person to question Michael's plans or decisions.

When it appeared that Michael may be going a bit too far or excessive, Tom spoke up. Often, drawing Michael's rage.

By the end of Part 2, it appeared that Michael had nothing but disdain and resentment for Tom. I think Michael was truly jealous of Tom.

Even though Tom had a mistress and Michael didn't, I think he was envious of the life Tom led.