I own one and have never seen any agression with the exception of him defending his cave.
If mine is out during day light he is scavaging for food... period. The rest of the time he is in his cave. At night (I have a diy moonlight) he will come out and swim around, and for all the time I have had him (9 months) I have never seen him be aggressive to any other fish.
dwarfs is right though, any fish that can fit in his mouth is food (at this point for me that is just fry- and he did a pretty good job of cleaning up the few fry I had in the tank.