Why does the gang keep an insane serial killer around? Because by doing so they're actually keeping him in check, in a way jail couldn't.
Nice Pete has a certain bizarre loyalty to them, and it appears as though mostly he hangs around the house all day blogging or writing novels- a drastic change from the violent rampages he apparently went on on a daily basis prior to his arrival in Achewood. He's never actually seen killing any sentient being during the run of the strip despite having many an opportunity to do so. (Although that hasn't stopped him from assaulting people or trying to commit murder.)
In this strip allegedly showing the futures of Ray, Roast Beef, Teodor, and Phillipe, Ray and Teodor are shown to die and Teodor has two white X squares to indicate the years that transpire after he's died. But Beef has a black X square after what appears to be an older and angry Molly at the end of a downward spiral...does this mean...?