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.