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)
Rambo: NOTHING IS OVER! Nothing! You just don't turn it off! It wasn't my war! You asked me, I didn't ask you! And I did what I had to do to win, but somebody wouldn't let us win! And I come back to the world, and I see all those maggots at the airport, protestin' me, spittin', callin' me "Baby Killer!", and all kinds of vile crap! Who are they to protest me, huh?! Who are they, unless they been me and been there, and know what the hell they're yellin' about?!
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.
Rambo: I could have killed 'em all, I could kill you. In town you're the law, out here it's me. Don't push it. Don't push it or I'll give you a war you won't believe. Let it go. Let it go.
John Rambo: Why are you pushing me? (00:06:40)
Rambo: For me, civilian life is nothing! In the field, we had a code of honor: You watch my back, I watch your's. Back here, there's nothin'!
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