His biggest mistake was volunteering to kill Sollozzo and McCluskey, which set him on the wrong path for life. As I posted previously: the very same suggestion he made for dealing with the aftermath of McCluskey's killing--feeding newspaper people on the family payroll stories about McCluskey's corruption--could have been used before the shooting, and would have thwarted McCluskey and Sollozzo without the need for bloodshed. (But then we wouldn't have had a GF Trilogy.)