I've heard that you open the last locker after you play thru the game a second time. However, I heard that if you open it the game crashes or something like that. Not sure if this is the backup version or th eoriginal. Could be the original since the game is buggy as hell. In short, I wouldn't open it if I were you.
I did it in V.2, it's pretty hard. You have to throw a shell so the guard turns his head to the right (his right) then run along the wall to his left, then snap his neck from behind. All before he turns his head back around. Also, you must kill him or you fail the test. It's a challenge, took a couple dozen trys, but you should be able to do it.
The Star Locker, don't open it!!! Everything will be fine if you do, you'll get infinite ammo and all the guns, BUT you don't get the code breaker and can't complete the game. This is fixed in the PS2 version.