As our roguelike-project is starting to take the form of an actual, playable game (which I still won't publically reveal until we're at Beta-state), let me talk about the philosophy of encountering monsters in the game.
Monsters need to be scary. That is the very essence of the word. Monster. When I DM an pen & paper campaign, a fight that is not a threat to the player-characters' lives is not a fight worth rolling dice for. I dislike success-porn. When playing a video game, my opponents better be a threat to me too. If not, what's the point? For me as a player to feel good about myself? That's a bit like punching toddlers, isn't it? This is one of the main reasons I like difficult games for. Playing a game to overcome adversary (actual, effective adversary, not just pretend-one), not just to get a pad on the back.
In the game a friend and I are making, you explore a cave filled with monsters, trying to find a way out. You start the game unarmed, empty-handed. As it is procedurally generated, no one, not even I, can say, when or indeed if you will find a weapon. While you're unarmed, every single monster the game has to throw at you can kill you. Don't get this wrong, the weakest beast in the game is weaker than an unarmed player, but with some bad luck it can get them anyways.
The game doesn't throw wave after wave of monster at the player. But each encounter is a potential threat to the current game, as permadeath is looming. This is supposed to make it scary when the player notices that yes, there is a monster in the next room and yes, it may come over here to eat me. In a fight, you should almost always have to consider running away as a viable option. Because monsters are scary. If you can go hunting monsters, they are no longer monsters but animals, prey, game. The term monster hunter is foolish. What you can hunt is no longer a monster.
This doesn't mean that the player doesn't get the upper hand over the first-level critters they encounter throughout the game. Find a staff or a sword and you can fend off the easier monsters effectively, albeit with a certain risk of getting hurt left. But then you run into the tougher monsters and just your staff won't cut it anymore. Be lucky and find a rag-armour or a shield to go with your sword or even a more powerful weapon and you might stand up to mid-tier monsters. But then there's the next tier waiting for you, further down in the cave...
The game will expect the player to think on their toes, utilize the layout of their surroundings, flee, lure monsters into each other, take passages that give them advantage and remain one step ahead of the horrors lurking in the dark or be devoured by them. That is the essence of threatening NPCs in a game. Those are monsters.