I've read of a guy putting four small octos, not sure of the species, kept in a fully stocked reef tank. When small they got along fine and didn't bother any of the fish or each other, but as they grew the larger octo picked off the smaller ones. Long story short, the octopus decimated the tank, killing every fish, if memory serves me correct 14 fish.
I believe that when certain species of octos, especially after feeding get harassed by fish or something else they will grab and kill what ever bothers them and just discard the item. Here's how I came to that conclusion: I would put five striped shoreline crabs in the tank, and my octopus would grab two or three and eat them up. He would finish them and spit out all the pieces of exoskeleton. The next day I would find the remaining crabs dead fully intact. I thought this was weird, so I decided to try an experiment. I fed my octo a raw shrimp that was too big to finish, so I would be sure that he would eat his fill. After about an hour I put a shoreline crab in the tank. He would push it away with his arms or use his siphon. After a couple attempts to push it away he grabbed it, held and killed it then released it once it died.