On the weekend, we had an incident at our table where the DM busted out a single bard at level 12, with NPC WBL. The party was level 12 with above average WBL.
This encounter is technically below their CR. However, the Bard proceeded to kill 2 party members in the same turn with Weird Words and (the word casting variant of) Celerity.
Also, stuff like this isn't really new.

The party complained that a single enemy should not be able to pull off something like that. Kill two party members in a single turn before anyone has a chance to act.
I'll shorten the discussion somewhat, as it lead to an interesting question in my head that I didn't really voice at the time.

Are there tools that are perfectly okay in the hands of a player but not in the hands of an NPC, or as part of a CR appropriate (or lower) encounter?

The arguement made by the party is that in the hands of a player, killing NPC's before they have a chance is okay. Because they are NPC's, the DM can just dig out more. But doing so to the player characters is unacceptable because then they are removed from the encounter. A particular player specifically seems to take this as a signal from the DM to not participate in the encounter, and admittedly does so rather personally. This player's specific remark was "if you throw a mailman at the party, you get books thrown at you."
Again, similar remarks have come up VS similar incidents.

However, in a world where the NPC's have access to the same stuff that the party does, where do we draw the line here? Especially when the player party grew up and trained in this world, knowing full well that other people like them exist.

This arguement will probably make a B-line for the 'save or suck/die' arguements, and partly stems from it. It seems that save or suck is okay in the hands of the party, but not the NPC's/DM.

Any thoughts?