A good GM is a GM that is ready, willing, and able to kill player characters. A bad GM is one that enjoys doing so.
I want to explain why a GM needs to be prepared to kill player characters and why it makes a better game for players and GM alike.
The ideal encounter
In an RPG session, the ideal encounter feels epic. The players feel like they did something awesome. In a perfect world, there will be at least one moment from every session that the players will talk about for months to come. The epic moments.
Epic moments make the fantasy feel real. They are the reason you are playing. The trouble is, that often as characters progress, the players figure out how to stay safe in whatever world the GM has put them. Throw undead at the players and they will make, steal, or purchase items with undead bane, holy, and other undead beating attributes. Put a lot of assassins into a game and players will invest heavily in perception and feats to avoid surprise attacks. Send dragons and players that don’t get eaten will go looking for flame retardant clothing. Too many wizards and the party may invest in counterspells and anti-magic enchantments.
However you run a game, it only takes a few levels for the party to figure out how to get the better of it. That process can be fun for both players and GM. The trouble is what comes after. The game sessions that follow can feel uninspiring or even boring. Once the players grow tired of easily slaughtering your hordes, they have nothing to challenge them. That loss of threat robs them of the feeling of being epic.
Make TPK a real threat
That boredom factor is actually not the reason why the threat of a TPK (total party kill) is a good idea. The reason the threat of TPK is so great is the thrill of surviving against the odds. Those moments when it looks like the party could be killed and they start to wonder if they should retreat – those moments feel like a threat. In short, the danger of death is thrilling.
When the party finally slays the dragon with a handful of HP left between them, that moment feels epic.
Those moments are good for the GM too. For one you yourself have no idea if the party can win. It can be as exciting for you as for the players to see if the monsters or the party will win.
There are other ways to make a game feel epic. Moments of crowning moments of comedy, heartwarming reunions, and tragic losses are part of the game. Even so, nothing feels quite so personal as “that one time we all almost died”. That’s why you, as the GM, need to create encounters that could kill player characters. At the very least, the players should think they might have a character die.
Ways to threaten TPK
One way to threaten TPK is to make sure you use tools like the CR calculator to select enemies that put the player’s luck to the test. If they are winning too easily up the CR. Add more monsters to the NPC party. Level up the goblins. Give the bad guys extra class levels in wizard or fighter. Plan ambushes. Create situations that put them at a significant disadvantage.
Make running away a valid battle plan.
If you threaten TPK consistently, the players will do what they always do, toughen up, and get creative. They will also get bored. Players are hard to please.
So do go easy on them. Give them some easy wins but let them worry that at any moment things could get tough. Every now and then set out to kill player characters with traps, angry mobs, ambush, fire, bad weather, poor luck, and hungry dragons.
When the adventure is over make sure the players have been reminded that when the GM smiles, it is already too late.