Running combat

Remember, "monster" is just a term for something that the PCs might end up fighting. It includes hostile (or potentially hostile) people as well as spirits and beasts and honest-to-goodness monsters.

Regardless of their nature, monsters have personalities—or at least patterns of behavior—that are more nuanced than just "kill the PCs." Every monster has an instinct. Most have a sense of self-preservation. Many have wants, needs, fears, or weaknesses that the PCs can leverage they're characters as well as monsters.

Portray your characters with integrity.

Consider what this monster—with this instinct and these motives, facing these PCs who are acting in this way—would do. Then have it do that. Hint at the monster's story and its inner life. Present monsters who might not actually want to fight, or who can be reasoned with, or scared away.

But also, portray monsters as monsters.

Try to capture the fear of facing down a hungry predator or a big angry beast. Play up the tension, desperation, and bloody consequences of a fight with another human being. Think about what it'd be like to encounter a hungry ghost, or an otherworldy Fae, or a demon made of boundless hate and jealousy, and try to convey that to your players. Make your monsters scary.

Let things burn. Don't protect your monsters, and don't protect your NPCs, and don't protect the PCs. If the fiction and the dice and your sense of what would happen all point toward injury or death, then go with it. Don't fudge things in pursuit of some grand story. Play to find out what happens.