An environmentalist-turned-hitman wants to retire, but he knows his controller, who is also his best friend and business partner, will betray him and have him killed.

This sounds like a set-up to the story but not the story itself.

The story would be the Hitman executing a well thought out plan to escape the business all while trying to evade his controller best friend, who is out to give him the ultimate retirement.

I do like the premise. I can see his best friend, the controller saying, “Why are you making me do this, you knew there was no way out when you joined the company” (That sort of thing)
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A: You need a very strong inciting incident (And that incident should be in the logline)

B: Stakes: Death

C: Inciting incident: The goal needs to be something the hitman would risk death to fight for.

Perhaps the hitman discovers he has a kid. (A daughter perhaps) Suddenly he realized he wants to be there for her.

But he can’t just quit his job, There are no ‘retired’ hitmen, and as much as he and his best friend care about each other, his best friend must go after the lead if he tries to escape the company. That is simply the code.

Good luck with this!!!

Possible Title: The Code