Depends on how good the evidence against them and what the crime is. The punishment might not be enough to completely stopping crime, but if it saves just one future victim, it's worth the deterrent effect.
I believe that those criminals and crimes which was subjected for death penalty will be judged fairly. They will undergo the due process and will not just be sentenced to death penalty. I agree with the death penalty especially now that people are so heartless. Just this day, I heard that a group of 19 years old were inquested for the crime of murdering a 32 year old security guard. It was so devastating that at that young age, people are ready to do such crimes without thinking and analyzing things.