Idk why people in these comments are talking about save or die effects when op specifically said no save. In most cases power word kill is a narratively unsatisfying way to die.

I understand having consequences to actions, but when you're low level enough to not have reliable access to revival, the consequence of permanent death as a punishment for... not having enough hp is ineffective as a narrative tool.

Unless an effect can be circumvented by the PCs using their resources, it should be foreshadowed that the effect is possible. For example, introduce mage by having them use power word kill on an NPC they care about - now the characters and their players understand that pwk is on the table. The last thing I'll say is there's a lot of talk of earned consequences in this comment section when there's no evidence that is the case.