Honestly I don't think (or want) an insane backstory, cause what makes Pete insane is that he's an absolute monster of a human who came from a very normal background (New Jersey).
Even then we can infer the journey that he took from stuff from the previous chapters:
- He choose to start kicking only to make things more challenging during his youth at a national, he still was able to win easily. Presumably he continued to win nationals later on until he became an adult or got bored & stop attending such tournaments.
- He stated in an earlier chapter (forgot exactly which) that the last time he was this excited was when he first entered MMA fighting. Presumably he breezed his way through the competition, found it lacking, and went onto the following.
- We saw him whooping the ass of an underground fighting champion, presumably he traversed the underground fighting scene of North America in search of a satisfying fight (probably using his winnings to fund his travels), found no worthy opponent.
- Then went on to live a normal life until he grew too much poppies & got arrested.
Edit: Also Tomahawk's mob boss sponsor stated literally at the end that he never wanted to fight Pete again, implying that he fought Pete at some point (presumably at some underground tournament) and got his shit rocked.