"Forget about Freeman" is the scariest moment in Half-Life 1
“Forget About Freeman” is the scariest level in the entirety of Half-Life 1, and it has absolutely nothing to do with conventional horror, aka monsters or a scary thing attacking you.
This is the moment where the entire situation at Black Mesa signals the future extinction of humanity, and the worst part is that the military, the people who were supposed to contain this entire mess, are beginning to realize it themselves.
The HECU did not exactly arrive at Black Mesa with a pathetic little security detail and a couple of rifles. These are actual military units equipped with assault rifles, shotguns, machine guns, explosives, armored vehicles, tanks, helicopters, and air support, sent into a research facility with the explicit objective of eliminating the alien threat and silencing the witnesses; they know what they are fightning.
The Marines are clearing rooms, setting up defensive positions, operating heavy weapons, and engaging the aliens with a level of coordination that makes them genuinely dangerous opponents. Sure, they are also trying to kill Gordon Freeman, but that is a separate problem entirely.
But here's the thing. As seen in Half-Life 1 and in Black Mesa Source, they are literally losing, even without factoring you, they are losing. The aliens just keep coming because it is an entire universe vs a few batallions. They do not worry about supply lines, ammunition production, transportation, reinforcements crossing an ocean. They are fighting creatures that can be teleported directly into the battlefield from another dimension, and their apparent numbers are effectively impossible to control through conventional military operations.
Humanity is essentially trying to fight an entire alien ecosystem, an entire universe, not a country, not a planet, a whole ass universe who keeps teleporting things. It does not matter how many bullets you fire if the enemy can keep appearing and has billions to spare, and it does not matter how many positions you secure if the battlefield itself is becoming increasingly unstable.
The wider Half-Life lore establishes that the Resonance Cascade has consequences beyond Black Mesa, with alien creatures and dimensional disturbances affecting places elsewhere in the world, even if the original game does not spend much time showing us the global picture. Black Mesa is the immediate battlefield, but the implications of what happened there are much larger than the facility itself. Humanity has accidentally opened a door into another world, and nobody has a reliable method of closing it.
Then you hear the radio.
"Forget about Freeman! We are cutting our losses and pulling out! Anyone left down there is on his own"
The situation has deteriorated so badly that Freeman is no longer the priority. The military is abandoning the pursuit because it has much bigger problems to deal with, and the people responsible for coordinating the operation are apparently beginning to recognize that the entire mission is going to hell. Their equipment is not enough, their training is not enough, and their numbers are not enough.
After this? You rarely see HECU. You are moving through a military disaster's aftermath. Destroying Black Mesa does not undo the Resonance Cascade, eliminate the existence of Xen, or magically restore the boundary between dimensions. Because the Combine upon seeing this disaster, quickly invade and defeat humanity in seven hours.