User blog comment:XGlass Reflection/Gaiden Chapter 0 Discussion/@comment-17563801-20131003190016/@comment-7418318-20131004073712

I think this likely is backstory of him before joining. Considering how careless some and how corrupt other members of the military police are, it's not that unlikely that criminals could get their hands on 3DMG (they even put such a scene of MPs selling equipment in the anime).

Ithink that's what happened here. Levi and his two buddies are really criminals who managed to buy some 3DMG on the black market and are now going to utilize it for some criminal purpose.

Petra said that Levi is rumored to have been a "renown thug" to Eren.