Get it drilled into their heads by decades of FPS gaming to want to kill every enemy that they come across
Design their setpieces specifically around this trained response from their playerbase
"What if I just run?"
All memery aside, I've been waiting the whole let's play for this moment, specifically - I've never played any of the Half-Life games, but I watched someone else play the Nova Prospekt level back when the game was new, and as someone who really likes giant bug monsters a la Starship Troopers
but gets annoyed that they're always typecast as mindless villains, seeing the bug monsters get a chance to fight on the good guy's side made this one of my favorite gaming sequences ever.
However, having not seen the rest of the game leading up to that level before, I was kind of expecting a bit more... build up to it? I always knew that you killed a boss antlion and harvested its scent glands to gain the ability to have antlions fight for you, but I guess I was expecting someone to explain what was happening beforehand, like "there's a big bug in that hole and if we harvest its glands we'll be able to tame the smaller bugs". Instead, you kind of just fight this giant space flea from nowhere, and then a vortigaunt runs up, shoves its organs into your hand, and says "the Freeman must follow" while giving zero explanation of why you're suddenly holding a bug's organs. It's fairly amusing, but I'm just kind of surprised how quickly you go from "shoot the bugs" to "the bugs are your buddies now".