Sure, let's assume government is obsolete. But there are still groups of people united by a common creed that will choose to employ aggression and violence as a strategy to compete for resources.

Humans do wars. Its what they do. Even when they had to mine for gold to fund them. Heck, many soldiers fought for no wage at all, just on the promise of a glorious afterlife. I don't think adding a new scarce resource for people to compete over (i.e. BTC) is going to fundamentally change that.