Very thought provoking.

The timing is significant in that this occurred while Michael was planning on killing Fredo. Tom's "betrayal" would have been much less significant than Fredo's, but also bear in mind that Fredo was a blood brother, which Michael always seemed to distinguish. We never saw the camaraderie and closeness between Michael and Tom that we did with Michael and Fredo and Sonny. Which would make it easier to off Tom in Michael's mind. How easy would it have been to add Tom to the Roth/Pentangeli/Fredo list of hits?

Having said that, I can't quite convince myself that he would have offed Tom. Taking a new job, while not the norm, wasn't an act of betrayal anywhere near that of Fredo's or Pentangeli's. More importantly, despite the desire to kill his enemies and the hardening of his soul, Michael remained pragmatic. He was going to have his blood brother killed (albeit for reasons one could understand, if not necessarily agree with). He was going to convince his capo to commit suicide. He was going to send arguably his #2 man on a suicide mission so that Roth would die. This alone pushed the bounds of trust. Adding to this the killing of his adopted brother for such a reason would crush morale, loyalty, and cripple the family irreparably from the inside out.

So what I think would happen is this - Michael would besmirch Tom's reputation as best he could, then apply significant pressure to Tom's would-be employer to not hire him. In short, make him an offer he couldn't refuse. Rinse and repeat as necessary. Eventually, Tom would either have to return hat-in-hand back to Michael or live in the manner Fredo did post-boathouse. Isolated and watched.