Hmm. I could see it existing, you'd just have to make it sick hard to actually get somebody's head off. You'd have to crit, make a successful follow-up attack roll against a different defense for some spiffy additional effect, then follow that up with an attack at a third defense for the insta-kill. And the bad guy might have to be bloodied in the first place.
What I'm saying is, you could probably make enough hoops to jump through that it's not actually overpowered for what it costs, but that's not the same thing as being balanced. The problem is that it makes combat more unpredictable, which strikes against the whole "expected combat experience" thing. So, it's going to have to be both expensive and usually not actually insta-kill if it's going to match up with WotC's apparent goals for 4E.