In The Godfather, Tom Hagen pushes Sonny to do the deal with Sollozzo because he believes it is the best way to avoid a gang war that would likely result in more violence and death for the Corleone family. After being released by Sollozzo, Tom is aware that Sollozzo is looking to take out Don Vito because he rejected Sollozzo's offer to enter the drug trade. Tom knows
that Sonny is interested in the deal, and he tries to persuade Sonny to accept it in order to avoid further conflict with Sollozzo and the other families.
Tom believes that accepting the deal and working with Sollozzo is the best way to protect the family and avoid a full-blown gang war. He knows that Sonny is already planning to kill everyone and declare war on the other families if push comes to shove, but Tom is trying to find a way to avoid this outcome. He knows that Sonny is inconsolable and on a war path after learning about the attack on Don Vito, and he is concerned that Sonny will not be able to control his emotions and will take actions that will only lead to more violence and death.
Tom is also trying to avoid bloodshed because he knows that Don Vito is already thought to be on his death bed and that the family cannot afford to lose any more of its members. He is aware that Sonny hit Bruno Tattaglia's son Phillip within no time at all after hearing about what happened to Luca Brasi, and he knows that Sonny is not going to be easily dissuaded from seeking revenge for his father's attack. However, Tom is hoping that he can find a way to persuade Sonny to accept the deal with Sollozzo and avoid a full-blown gang war.