Deterrent. The death penalty as a deterrent must be the reason why the crime rate in the US is so low compared with countries that dont have death penalty. Deterrent against crime. Cool story bro.
I definitely detect sarcasm here...at least I HOPE so.

Some in Congress and the military think detention and interrogation of enemy combatants without due process is necessary to protect the country. I'm not sure I agree, but I don't think they have motives other than protecting the country from harm. I definitely don't believe the motive is to frighten regular citizens. Standard civilian law and punishment are sufficient for that.
Oh really...then you may be interested in some of the Orwellian characteristics of what the DHS considers potential terrorist suspects (even the Culturally Marxist ACLU, suspiciously, has decried the NDAA:
7 Days or more food supply
Belief in Conspiracies
Buying Flashlights
Missing Fingers
Own Precious Metals
Owning guns and ammo
Paying Cash at Hotels
Ron Paul Stickers
Occupy Activists
Tea Party Activists
Texting Privately in a Public Place
I bet you exhibit at least one of these attributes...and these are just a few examples. You can arbitrarily be labeled a suspect and be carted away without any due process.
Dr. William Pierce said that you can not try to explain to a lemming that following the herd will lead to his inevitable demise, you must convince the lemming that the herd has taken a new path.
Just remembered this line, don't know why...
That is from George Orwell's book 1984. If you haven't read it, consider it or get the audio book or the movie.