Good point Capri Michael did let his man's murder go unavenged indeed
and Michael didn't know “Alive -- Pentangeli is alive”

There is also the insult of the C-note
Quote
Extracts:
Carmine Rosato: It's a lucky C-note for our new deal.
Pentangeli: I don't like the C-note Rosato -- I take that as an insult

I doubt Roth would have sanctioned Pentangeli's murder but perhaps Roth did?
Rosato brothers small potatoes!

even though Michael was to have been killed in Havana, the Corleones would have lost their muscle, their Caporegime so that Neri and Rocco can't retaliate swiftly

The Rosato brothers could have taken over and well established themselves in New York, by the time the Corleones have regrouped [if] following all the chaos ensued if Michael had been killed in Havana and no muscle