For me I always thought Clemenza was happy to take out Carlo. It seems that it would have been personal to him because of him being Sonny's Godfather (or so I've always thought). As far as Tessio is concerned though we never saw him hit anyone I always thought he took care of the driver when Micheal took out Sollozo and McCluskey. Also it seems that real life mob history on occasion with high profile hits a capo has been given the contract.