Have you ever been afraid of security breach into your home?
Well is the answer is yes let's just blow your mind with this easy to install zombie killer!

We could go for a 1x1x2 death trap but since we want securityo ¦ Let's go for a 2x2x5 deathtrap!

First you need a 2x2 entrance for your home. secured with a door. If the door get breached don't panic because 1 square after the entrance is the start of your death trap.

I used wool to mark what you need to do:
Red is for Redstone
Orange is for repeaters
And white is for levier

You just need to connect the two sides of the death trap by an under ground tunnel so the levier will activate both sides at once.
Note: I used a repeater on the tunnel to the right side to boost the active radius of the redstone so either you need to give 1 more second to each left side's repeaters or put one more on the left side.

I used glass to make the parts of the floor which is moved by the pistons to let see through. It is not required.
Actually if you use that in SMP to kill people I strongly recommend you to use uniform ground. (As a reminder if you cook cobblestone it will get back to it's natural look). If you play is SMP and do not have access to lava you may consider using a very deep hole to insure the death of the foe.

Note that you can also use one levier to activate not only the side toward you but both sides of a hallway, leaving something like a 3x2 or 5x2 safe zone for you.

If you go for the disguise of a simple hallway I strongly recommend you firstly not to use wool (it was there only present as demonstrative purpose) and to disguise the trap with something like a redstone activated lamp to discourage people activation the levier in the first place.
I.E.: Why anyone would just enter a house and switch off all the lamps?

Here you can see the red, orange and white wool that composes the power circuit.

Up and down levier with glass panels.