I think you are being too harsh! on Tom

Sicilian Vito the most experienced, long reigning top Mafia Don “wasn't on the lookout for the irrational need--compulsion--for vengeance” by Sollozzo after Vito's refusal to help with Sollozzo's narcotics trade

As Vito said in the Garden scene Sicilian “Santino was going to have to go through all this” ie: Don in training didn't see it coming either

Then, what chance Tom “non-Italian uninvolved in the muscle end of the business” with his lawyer “background and training” and not a wartime consigliere have, to see it coming

So how “old Genco [this wonder! consigliere with all his heroics!] would have smelled a rat" and “Tom didn't”

I doubt “Michael blamed Tom for Sonny's death” even if “Tom had some culpability in Sonny's death” as unltimately and undoubtedly "Sonny's uncontrollable temper and poor judgment sealed his fate"