Sounds like a question unlikely, at this remove, to ever be answered. Historically, Lenin and Stalin first purged other Marxist parties - e.g. the Mensheviks and SRs. Then Stalin, in the early 30s, purged "Old Bolsheviks" who might have enough of a following to become a threat to him. So it would not be unprecedented in Communist history for the Castros to have purged a competitor within party leadership.
For that matter, I sometimes wonder - based on zero information - whether shuffling Che' off to foment revolution in South America might at least in part have been exiling Che' to a hopeless cause and eventual death. Not that I've ever had any sympathy or liking for Che' or the Castros.