Colonel Trautman: I don't think you understand. I didn't come to rescue Rambo from you. I came to rescue you from Rambo. (00:45:25)
Sheriff Teasle: Are you telling me that 200 men against your boy is a no-win situation for us?
Colonel Trautman: If you send that many, don't forget one thing.
Sheriff Teasle: What?
Colonel Trautman: A good supply of body bags. (00:48:05)
Sheriff Teasle: Whatever possessed God in heaven to make a man like Rambo?
Colonel Trautman: God didn't make Rambo. I made him.
John Rambo: Why are you pushing me? (00:06:40)
Chosen answer: Because it's a knife that someone else is holding and can use against him. It's common for people in dangerous professions to regard their weapons as tools. For example, police officers who commit suicide by handgun rarely use their service weapon, but rather a personal firearm.
Captain Defenestrator
Additionally, I believe it is made clear during that scene that Rambo had previously been tortured at some point, and one of the tools used was a knife. So the combination of being restrained and having someone approach with a knife caused him to basically have extreme PTSD and so he attempts to escape.
oldbaldyone