I never found it credible that Sonny witnessed Vito killing Fanucci. Vito was ultra-careful to assure that neighbors witnessed Fanucci leaving the apartment, and that no one witnessed his killing Fanucci.

Ntra la porta tua lu sangu � sparsu,
E nun me mporta si ce muoru accisu...
E s'iddu muoru e vaju mparadisu
Si nun ce truovo a ttia, mancu ce trasu.