WASD or Arrow keys to move.

Click on a villager (Black Cube with Yellow face) to kill them, but only if the cursor turns to crosshairs over them.

If a guard (Black cube with Blue face) sees you, he will be angered and shoot at you.

If a villager sees you, he will run from you and become Afraid. If and Afraid villager runs into a guard, the guard will get angry at you.

When all the villagers are dead, you win!