I've found enemies in shooters to be really weird just idling until you show up in their vincinity. They should know you're there and be coming for you or hold defensive positions. Or they could be unaware and unprepared, running for cover and their guns when they see you.

That was twenty years ago. Reading this article made me wonder whether it's still the same today. On second though it's probably about offering a game that allows people to shoot enemies in a satisfactory way. Having enemies too smart wouldn't allow killing hundreds of them because they'd get you first.