Kind of. If there is a particularly beefy opponent, maybe not. I guess you would have to investigate the average HP per CR, to really look into it.
Does the fighter only get one of these attacks a day? I would like the spell to be worth consideration of the one and only 9th level spell slot a caster gets a day. I'm not sure it is, as written. Even setting aside the whole 'does the player know the current HP of the monster issue (personally, I would give the PC a pretty good indication), it is only useful as a 'finishing' move for most foes a 17th level plus caster would face. Is that enough? I don't know. How often does that come up? How often does it come up where you can't just spend another round or so with party dealing 100 damage to the target?
I guess it comes down to the fact that there is a highly asymmetrical relationship between monster & PC hit points, thereby making the spell very deadly for PC's much longer than for Monsters. Personally, when I get a game up to that level and it was problematic, I would just have the spell do 16d6 psychic (or maybe necrotic?) damage with a Wis Save for half on creatures of greater than 100 hp.
I suppose when I think about it, I'm looking for the opposite. The idea that a high level wizard has a spell that would allow them to automatically kill 1 BBEG (whatever that might be, such as the 1e dragons given in examples above) is way too overpowered from a world-building standpoint.
To consider it a different way, with a spell as powerful as it was, an evil wizard villain could simply teleport in (or use some other means of quick transportation), and kill the most powerful creatures of the land, one day at a time. Is it all that more deadly than an assassin with poison? Without a saving throw, I'd say yes.
To be fair, I generally have problems with the way high level magic scales in campaigns, and for that matter, high level adventurers. While 5e has reigned things in a bit with bounded accuracy, etc., it still roughly equates "high level" with "high hit points," both in the amount a creature has, and the amount an attack deals. Monsters have to have a higher hp value, because it's often 4 (or more) against 1, eventually. Since they stopped allowing spells to be disrupted in combat, and eliminated most of the risks associated with a lot of the spells, the balance has been off even more.
Within the context of a combat it's OK, but I just think there are better ways to potentially approach it. Regardless of that whole aspect, though, a spell that causes instant and unavoidable death still seems to be more power than I'd like. So as a whole I've never really been a fan of the spell in any edition. So with that being said, the 5e edition works pretty well for me from a design standpoint.
In terms of it being the one and only 9th level spell per day, I can certainly understand the desire for things to be balanced between spells. But I've never really worried about the level of a spell per se, or if the wizard's spell is just the "finishing" spell as [MENTION=12731]CapnZapp[/MENTION] mentions. I've always viewed and played D&D as a group thing, that the group succeeding or failing is what's important. Not necessarily who did what with what level spell. If
power word kill allows me to prevent the party from being subjected to additional damage or danger by taking out the last 3 hp of the BBEG, then cool. That was the point - I don't really care how much the others did or didn't do (as long as the spell worked). Although if the spell didn't work, that's just as fun anyway. It just leads into a new circumstance that has to be resolved.
The other thing I should mention is that we've never been focused on the numbers while playing, and a given combat is almost never the focus of the game. Combat is an obstacle on the way, not the focus. And I rarely have BBEGs. At least in the BBEG mold. There are key villains, but defeating them isn't always (or even usually) a combat scenario. And most of the time, they are just at the "pinnacle" of another layer. More importantly, they are usually a part of a much bigger organization, and it's kind of like toppling a dictator in a corrupt government, there's always somebody waiting to pick up where they left off. The types of challenges that characters in my campaigns face are often quite similar from about 5th level and higher. We rarely reach more than 12th level anyway, but even still, they are facing beholders, dragons, demons, liches, and all the fun stuff. It's far more satisfying when a party of 6th level characters devise a plan to destroy a lich or slay an ancient dragon.
So our circumstances are probably quite a bit different than a lot of the folks that don't like this particular spell because it's not powerful enough. I personally think it's a step in the right direction myself. Oh, and it is possible to disrupt spellcasting in my campaign...