I also thought that they are scavengers until recently.
I bought 5 medium size Amanos from Aquariums West couple of months ago. There were 5 threadfin rainbows in the tank for almost a month before I introduced the Amanos. I added 5 more threadfin afterward for a total of 10. I lost at least 1 fish every day or 2. After losing 6 threadfin rainbows and 5 Endlers, I finally got raid of the Amanos. No more death since.
Have I witnessed the kill? No. But with the amazing strength of these Amanos, I'm pretty sure they are capable of hunting down these smaller fish if they get hungry enough.
I have half a dozen amanos in a tank with community fishes including guppy frys, pleco frys and galaxy rasboras crammed into a 26 gallon with no issue. Maybe its a matter of how much you feed them? Or personality? Not sure..
I think just like with other creature, you get a more aggressive one sometimes. I have one huge female that will actually jump on my hand and pick at my skin every time i put my hand in the tank. I've never seen her on the plecos or other fish, but as soon as my hand goes in, she's on it, and trying to eat me alive. lol I actually have to shake my hand to get her off, and let me tell you it doesn't exactly tickle. Not a big pain either, put I could imagine it would be a strong weapon against any other fish in the tank.