The first & most important question is about setting; "what is this for?" Is it for a hero arena, an altered melee, a 5th-race addition, a MOBA, etc?
I'll move forward assuming it's a 5th-race addition/altered melee:
You have done right by drawing comparisons to other abilities in terms of balance.
Of the 24 Hero Ultimates, there are only 3 that are 'single-target'; Charm, Doom, and Transmute. Each of them 'kills' the unit with some bonus (one way or another; Charm removes it from the enemy ('kill') only to give it to you; Doom does DoT but always kills the target unit, giving you a Doom Guard; Transmute just kills, giving gold). I believe it's important that this is the case; in a game like Warcraft 3, the units are a lot more important than in a game like, say, Age of Empires or Red Alert. So if you have a single-target Ultimate & you're trying to fit game-balance, I'd say it definitely needs to (")kill(") it.
In that sense, Finger of Death is fine. Really the only question then is: "is it enough? Should it also do a secondary effect* (like all the others), or instead be balanced (mana-cost/cast-time/cooldown) to compensate (I.e. lower mana cost/faster cooldown than the others because all it does is 'kill')?"
That's up to you. Personally, while I find Finger of Death a tad boring, it's definitely 'cool' and impressive, & definitely fitting to Warcraft.
*Secondary effects might be something like a temporary debuff to all nearby enemies (i.e. they just say their friend get ASPLODED, they are a little freaked out right now). Or a buff/heal to all nearby/near-to-casting-hero allies (i.e. they just saw their leader ASPLODE some worthless whelp; they are pretty jazzed! //or// the "finger of death"-like spell kills a target & spreads his soul out to your guys; like a mass-"Life Drain" or something)
(actually that last one is kinda cool... Gonna have to use that somewhere)