Even if you believe that some crimes are so terrible that the people who commit them deserve to die, there are several good reasons to abolish the death penalty.
First, we make mistakes. So far, about 160 death row inmate have been exonerated, and it feels like every year another 1 or 2 are exonerated. That accounts for about 2% of death row inmates. Some estimate the real number of "innocent" people on death row might be around 4%. The 2% of exonerees were basically people who had ironclad DNA evidence vindicate them. Other innocent people have probably been sentenced but may lack that kind of evidence.
The death penalty is also unfair in many, many ways.
Racially--a black defendant is about 16 times more likely to receive the death penalty than a white defendant who is convicted of the Same crime.
Economically--there is a strong correlation between whether a person convicted of a capital crime receives the DP and how wealthy they are.
By gender--in caifornia for example, about 10% of men convicted of a capital crime receive the death penalty while only 1.5% of women do.
By geography--death penalty trials are very, very expensive because they are longer, require more attorneys/experts and more jury vetting. Because trials are paid for by the county where it is being held, many smaller, poorer counties are less likely to pursue the death penalty due to the cost. So, a person who commits a capital crime in Los Angeles is more likely to get the death penalty than a person who commits a capital crime in, say, rural Montana.
Finally, the death penalty is expensive. On average, sentencing someone to the death penalty and carrying out the sentence costs about three times more than sentencing someone to life without parole.
So, while I probably agree with you that some people deserve to die, we as a society cannot dispense that form of Justice fairly, or even correctly, and it is very, very expensive. For those reasons, I think the death penalty--though perhaps morally justified in some circumstances--is not worth it in practice.
(Most of my sources come from Michelle Alexander's "the new Jim Crow")