Why did he go to prison?

If there had been a sequel I would hope that tells it?

My guess is either he was innocent and framed or got a really bad rap like Nick Cage in Con Air or Hamid Khan in Oz, like maybe he killed someone in self defense.

No way he could be a real criminal he was too heroic. If I was the governor in the fictional world of that movie I would have pardoned him.

One scary thing about this movie is that the character was 42 years old (I always go with the actor's age if the character's age is not told), had been in prison for five years and you do not get the best food or exercise equipment there. So what we saw in this movie was a middle aged over the hill not in his best shape version of the character, you have to wonder how superhuman or "metahuman" he must have been in his PRIME. Prime Bone would be TERRIFYING if middle aged has been Bone could be so BadA$$. My guess is Prime Bone would have had an effortless or almost effortless win against Price while in the movie Price did some damage to him.

You wouldn't go to civilian prison for unauthorized absense.

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Every person that served can be called a veteran, but not every veteran can be called a Marine.