The african butterfly fish will eat any thing that will fit in it's mouth at the surface, and beat the crap out of other surface fish as well. it gets about 4", so it will fit in a 10G. keep in mind it is usually only agressive to surface creatures, and any fish in the middle/bottom will be largely ignored.
well.... at liveaquaria they have some dusky panther gobies, they look nasty. it says 50G minimum, but they only get to be 4" long, and they often don't get the minimum tank size right.
but I still think your best bet if you want an antisocial monster for a 10G, a butterfly fish is the ideal choice.
you could also do a SA bumblebee cat, they aren't aggressive, but they will eat anything that cat fit in their mouths.