Sergeant Hatred has been with the Peril Partnership the whole time.
When he was first properly introduced he was intelligent and capable, a bit silly and certainly no Brock but still someone not to be taken for a fool. Nowadays he just doesn't seem to be as capable for seemingly no reason. But he's been a high ranking member of the O.S.I. while being a double agent for the Guild, so it wouldn't be out of character for him to have been a triple agent for the Peril Partnership and still be with them now. Everything he's done to prove his loyalty to the Venture family, as well as his declining capability, has all been a ruse,
[b]Or[/b] he got fed up with how Doc treats him and at some point has gone over to the Peril Partnership. I think that's less likely than my original theory, though.