I am against the death penalty for several reasons. The biggest reason I oppose the death penalty is because innocent people are occasionally killed. The petition mentioned below is to investigate whether or not the person is innocent or not. More recent information points out that the person maybe innocent of the crime he is to be killed for.
I hate the death penalty it's just wrong. First innocents are occasionally killed, how many people are killed for crimes they did not commit is unknown. A book called In Spite of Innocence notes that between 1900 and 1992 there were 416 documented cases of people convicted and given the death sentence who were innocent. In at least 23 of those cases, the person was executed even though they were innocent.
Can list a lot of reasons like racist (more minorities than not), punishes the poor (more poor than not), cruel (It is premeditated murder) but my reason is that the person maybe innocent. Even if there was a way to prove with %100 accuracy that the person was guilty, I still would be against the death penalty. I believe people should get a chance for redemption and atonement. Killing a person will not undo the crime and I'm against all types of killing. Basically my morals are against it.
Though I do find it fascinating what crimes are supposedly deserving of the death penalty. Old Testament - Adultery and Homosexuality. Medieval Europe - witch craft, unofficial translation of the bible, sacralige. Romans - infidelity, theft, bearing false witness. Babylonians - wrongs at work build a bridge and it collapses killing someone was cause for death. Amongst other crimes. The means of execution were pretty brutal depending on the time there was crucification, burying alive, beaten to death, stoning, burning on a stake, suffocation with excrement (Teutons 100 AD), starvation, etc. The tools of execution were also brutal and varied guillotine, The Wheel, Firing squad, Noose, electric chair, gas chamber, etc.
More info on opposing the death penalty: http://www.amnestyusa.org/abolish/dp_qa.html