I don't usually do this, but this one is actually pretty clear-cut.
Marmaduke was captured by animal control. Once placed inside the pet shelter, he slaughtered the entire staff and freed the rest of the dogs, leaving the cats and other animals to starve to death in their cages.
Also, it is brought to our attention that particular residents of Ontario, Fredericksburg, and Syracuse either lead painfully hollow and meaningless lives, will write letters to basically any goddamn thing in the newspaper that asks them to, or have been fabricated by Marmaduke creator Brad Anderson as filler for his weekly spacetaking feature.