Berzerk is a classic!  Here is the introduction from the instruction manual: 

"The Astro Date is 3200 and you are the last survivor of a group of earth people who came to explore the planet Mazeon.  Soon after landing, you discovered the planet is a dark, apparently uninhabitable place.  But by then it was too late to turn back because your space craft had been destroyed by Automazeons.  Now you are a prisoner here.  You are trapped in a maze where even the walls are death to touch.  Grim robot thugs known as "Automazeons" stalk you relentlessly and you must systematically pulverize them with your laser gun before they eliminate you with theirs."

You can move your character up, down, left, right and vertically.  You can also shoot in all of those directions.  Your goal is to shoot as many robots as possible without touching anything.  If you are shot, if you touch a robot or if you touch a wall...you die.  You score 50 points for every robot that you shoot.  If all of the robots are destroyed before you leave the room and move to the next room, you get a bonus of 10 points for each robot that was in the room when you entered.  In some game variations of the game, you also must face a giant smiley face called "Evil Otto".  You can not kill Evil Otto, but on some game variations, you can shoot him and make him start back from the edge of the screen; other game variations, you just have to outrun him.  Some game variations you get extra lives every 1000 points.  Some game variations you get extra lives every 2000 points and some have no extra lives.  Berzerk was a very popular game when it came out and there are a lot of copies out there.  You should have no trouble finding it at a very low price.

Screen Shot from Instruction Manual