It is. I hate how you can't tell which ones are dead and which ones aren't.
Well, the normal zombies didn't scare me. They're desperate cries for euthanasia were greatly disturbing, but at that point I had already dealt with enough standard headcrabs and zombies to hardly even be phased by them.
Then they brought out the poison headcrabs. They had a scarier look, but it was the rattling sound that brought out the terror. Worse than the fear were the problems they brought forth. Knocking you down to one health temporarily made all other threats even scarier, also the zombies would THROW the damn things at you. Even so, they didn't legitimately terrify me.
Then, out of nowhere, they introduced the fast headcrabs. Alone, they were minor annoyances, but attached to zombies they were nightmarish. It was the speed mixed with the damage and the unpredictable swarms they came in. But the screams, oh god, THE SCREAMS. It's like someone running the teeth of a furious, rabid Tasmanian Devil across a chalkboard. A chalkboard IN YOUR MIND.