Question: Which one of the thousands of steps contributed to his death?
Answer: Every one of them.
Well, no, not one of them, he left with the plan in place already. He knew what the plan was, "When ‘X’ happens, ‘Y’ is the next step. ‘X’ is reaching Pacific, ‘Y’ is pulling trigger.
So answering your question, I was zero percent at fault for the choices they made. They did not have to choose to pull that trigger. Sure life might suck dirty scab filled balls, but not one person in their life made the choice to pull the trigger but them.