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.
Talking about the evidence, in normal circumstances the judges or the jury award death penalty only in cases where they are 100.000% sure about the guilt of the accused. Even if they are having a 0.001% doubt regarding his guilt, they will never consider the death penalty. In the past some innocent people were framed for fake charges, but I don't think that it is happening right now.