iserith
Magic Wordsmith
I am not sure I am understanding what you meant here. You seem to be using "would" and "could" in a somewhat (to me) unusual meaning or context. Are you saying that what you decide the monster does is, by definition, the one and only thing that the monster could do? I mean, I suppose...
If, on the other hand, the monster could do (in the sense of "in the abstract is capable of") any of several things, then the fact that you managed to choose one of those indicates to me that you in loco monstri do, in fact, know what the monster would do.
It's kind of like saying "It's what my character would do" as if there's only one thing they will do in a given situation. (Often that's used to justify why someone has chosen something that annoys everyone else at the table.) I always say "could," meaning that, sure, they could do that, but they could do this other thing as well and it would be just as reasonable. The same goes for monsters. Some might say that the mindless zombie would just go for the nearest target. I say, sure, they could do that, or the zombie could lurch toward the armored cleric, stumble on this here difficult terrain, and end up next to the wizard who it then attempts to slam. Or it senses the cleric's holiness on some level and avoids him in favor of the godless robe-wearing wizard. Or whatever other reason I might come up with in the moment.