The death penalty seems as if it's only useful for bargaining in the courts after the crime has been committed. I don't think it's a good deterrent for several reasons: (a) general lack of knowledge about state laws and in what circumstances you can get the death penalty; (b) death ain't really all that bad for some people if they're facing it in a relatively humane way, there are worse ways to die than by potassium chloride injection; (c) some criminals just wont care about death or only kill because they think they can get away with it.
Though I don't think the current judicial system meets the requirements to be able to determine whether a criminal should face the death penalty in most instances. I wouldn't want 12 people deciding what I have for lunch let alone whether a person is guilty of murder or not.
Neil deGrasse Tyson has talked about being thrown off the list of potential jurors because he asked why the judge read that the defendant was caught with 2000mg of cocaine and not 2g of cocaine. I think he also said there was another occasion where he wasn't accepted because they asked what he did for a living and he said he taught Astrophysics and courses like "Evidence and Reason" or something along those lines.
On another note, we have to watch what we say here. I know on this forum a debate about gun regulation could go on and on so try not to say anything that will spark a debate