if i relied on someone to watch over and protect my family, or anything i held dear to my heart, i would have very high expectations of that person. if i were michael i would most certainly be bitter against tom for kays abortion. michael loved that baby and wanted nothing to happen to it. he was so protective of the baby and kay that he would not even allow her to go out grocery shopping. tom manages to prevent kay from leaving the premises, on michaels orders, but how the hell does she manage to have an abortion under the supervision of tom. i see that as a huge fault. michael plainly states before he leaves that he is trusted tom with the lives of his family, HE FAILED in my eyes.
i always thought that michael was never fond of tom. although very subtle, when kay first meets the family at the wedding in GFI, he gives tom's background and says, "He's a good lawyer. Not a Sicilian, but -- I think he's gonna be consiglieri."
i interpret that as the first sign that michael was never really accepting of a non-sicilian so involved in the family business. then at the end of GFII, with all the brothers sitting aound the table getting ready for the B-day party, we see michael a little irritated with the fact that tom was planning out michael's future with his father.
TOM-Now you don't understand but, uh, your father has big plans for you. Now many times he and I have talked about your future.
MICHAEL-Talked to my father about my future? My future.
TOM-Mikey, he has high hopes for you.
MICHAEL-Well I have my own plans for my future.

i may be reading too far into this, but thats just what i think.

robert