Opinions vary. Read widely and then come to your own conclusion.
If you need help learning how to search, including how to choose good search terms, go here and scroll down to the links under HOW TO SEARCH THE INTERNET:
The death penalty lowers the crime rate only for those specific individuals who suffer the penalty.
Before the death penalty can be applied, the perpetrator must be caught, tried and convicted. The probability of each of these is far from certain. For those who intentionally commit the crimes, there is a relatively common belief that they won't get caught. Those, who do not intend to do the crime, are usually so emotionally wound-up that they don't think about the consequences.
I searched Google under the key words "death penalty" to get these possible sources:
There are many other sources available.
In the future, you can find the information you desire more quickly, if you use appropriate key words to do your own search. Also see http://hanlib.sou.edu/searchtools/.
I hope this helps.