My feeling as well. Gnolls have struck me as being savage and rather... brutish. Not subtle. They swarm the enemy and rip it to shreds. That's even part of their racials - extra damage for adjacent allies.
Skulking about and striking from the shadows seems much more patient and methodical. It's just not the image that the gnoll presents. This is more in line with a Bugbear, who are all ready portrayed as stealthy killers who even the brutish ones are keen and crafty (hence the predatory eye).
I'm still a bit confused about the complaints here. The nature of the Gnoll assassin feat seems to fit them well as vicious hunters - when they kill one prey, they are invigorated by doing so and immediately join back up with the pack to go after another. Nothing about that seems off.
If the issue is that gnolls have a feat at all - so does every other PC race. (Except Genasi). I'd say many other races are far less suited for assassin then the gnoll - the feat is there for the occasional gnoll that is an assassin, not there because all gnolls are assassins.
And even then, I still think that the ability to mimic voices to lure unsuspecting people into danger is a very good indication of a cunning predator, rather than relying on simple brutality alone. Compare to orcs, which really are all about brute force - Gnolls are much more presented as hunters and predators, and I think feats like Butcher's Lure and Gnoll Tracker definitely support that.