To be fair, you can "hidden blade" kill bears as Conner back before you ever meet Achilles. The animations are what they are and a character's attire has nothing to do with what animations they chose to give them.
It's one of my peeves about this game, actually. You can go downstairs and put on the prisoner outfit again, and still be doing hidden blade or other such kills. It makes no sense, but ... *shrug*
For the record, they recycled the kill animations from previous games.
Personally I don't even mind the kill animations. I just wish they'd give us an option to turn off the cinematic kill camera so I can spin the camera to see my pursuer coming at me while I'm in the middle of the kill instead of panicking in the last available second to try to find my pursuer after I'm done with the kill.
PSN: KazuoMitarashi / Currently Playing: AC3 (3rd Prestige), Borderlands 2 (Siren), DmC, SSBB, Zelda, and Halo 4