No, sorry to burst some people's bubbles, but this is not original.
Who cares though, it's concept is something I've always enjoyed, and it's what you do with a concept that makes it unique.
Also, to put at ease some people's doubts of "overpowered" people coming into effect because of this, I suggest that you add in a consequence of gaining many kills/going on a frenzy.
One suggestion of this would be to have it so that the more power you've obtained from killing, the faster it decays. This leads to being able to become overpowered for a short duration, something that many games implement. Of course, it requires the skill to achieve that level, so it is not unfair at all.
What i suggest you do is rather then having it as an instant absorption of the powers, rather let the player have a choice.
I'm going to take a concept from Bioshock 2 multiplier here, where a player, after killing another, can go up to the body and "research" it, which gains increased damage against that specific idea. The problem is that this research takes time, so you have to decide if it's worth risking being caught off-guard. If we apply this concept with power-absorption, we can lead to you have to physically go to the body and spend a few seconds gaining it's power. This way, not only is your player presented with a unique choice within the game, adding in the fact that it decays faster the more you have presents a unique way of playing.
"do you risk going for another power grab, only for it to decay faster, or do you leave it and use what you've got"
now, we can explain this further. Let's say we also make it so that anyone can take that power of themselves. The choice expands.
"do you risk going for an other power grab, only for it to decay faster, or do you leave it and wait to see if someone else goes for it in order to catch them"
With this, you have created a rather interesting mechanic in your game that can lead to many tactical decisions within.
With only one implementation, you have created many unique ways to play.
Anyways, that's my suggestions.
If, at any point, what I post is hard to understand, tell me. I am bad at projecting my thoughts into real words, so I appreciate the knowledge that I need to edit my post.
I am not a professional writer, nor a professional game designer. Please, understand that everything you read is simply an opinion of mind and should not, at any point in time, be taken as a credible answer unless validated by others.
I do take brief bouts of disappearance so don't worry if I either don't reply to you or miss certain things. I am quite a lazy fellow.