Pick up the crowbar of research scientist Gordon Freeman, who finds himself on an alien-infested Earth being picked to the bone, its resources depleted, its populace dwindling. Freeman is thrust into the unenviable role of rescuing the world from the wrong he unleashed back at Black Mesa. And a lot of people, people he cares about, are counting on him.
If you want to make a Zombies, Combines, Antlions, or anything or anybody else hate or like you, get this map and it will show you examples that you can learn from.
Another test map...
Valve developer community
There's no better place for learning about this stuff