it was probably becuase we all from time to time find ourselves in situations where someone has an advantage over us. Our instinct is to say "Oh yea, well you forgot about this". It's a defense mechanism. Tom was just saying to Sollozzo that you may have killed the Don and you mayhave me in your clutches right now, but wait till Luca gets ahold of you. On the other hand, it could have been the lawyer in Tom trying to be explicit about the terms of the deal Sollozzo was proffering. You get the Corleones to back off and we can all work this out. Tom might have simply been advising Sollozzo that the Luca variable was out of the Family's control.