I think Sonny would have caved into the pressure of drug dealing for two reasons : first he wanted in all allong anyway and second, the first time Hagen told Sonny "if the old man dies you make the deal" SOnny's rsponse was "thats easy for you to say he's not your father. The second time Hagen told Sonny to hold off and listen to what Sollozzo had to say Sonny agreed "to wait." It was MICHAEL who then said "We can't wait" so perhas Sonny was aware his temper had caused the whole mess and may have been more receptive to Hagen's suggestions.