I don't want to kill them. But I want them to be afraid.

I don't necessarily want to kill all of them, but, you know, putting a metagaming mage in an antimagic field, a min/maxed flyer in a little cave, no flanking plus some poisoning to reduce sneak attacks...

I'm just done with people showing up with "broken builds" they found online and totally legit stat blocks that they definitely rolled.

i TPK'd a party whose solution was to kill everything they disagreed with and it definitely made the game better with the next set of PCs they rolled up.

I just died in my last session because the rest of my party didn't believe the Lawful good paladin...

Do not be afraid to kill your PCs. Not egregiously, via Rocks Fall Everyone Dies, but the world acts and reacts. If your PCs are trigger-happy murderhoboes, they might find themselves on wanted posters with a significant bounty. Theirs is ostensibly not the only adventuring group, and nobler heroes might take it upon themselves to prepare accordingly to deal with the threat to peaceful communities the party represents.