Capital punishment is nothing but revenge institutionalized. It is dead wrong; no pun intended. Now, before I get accused of being a bleeding heart, I will tell you; if you are being attacked, you have the absolute right to fight back in any way you need to in order to end the attack. I would even support your decision to use deadly force to stop vandalism or theft in almost all cases, since property crimes are stealing your life and security. Death at the hands of the victim or a rescuing hero at the scene of the crime is a fair outcome for someone who has decided to initiate force. However, once the criminal leaves the scene, there will always be some doubt, somewhere, as to guilt.
No government anywhere is so pure that it should be trusted with the authority to decide to kill someone as punishment. Once a person is caught and charged with a crime, all the government cares about is conviction. The fact that many defendants are found "not guilty", despite the best efforts of the jury-tampering judges, shows that the system can still work. However, no jury is above being influenced by a judge or prosecutor who is looking to advance his career by being seen as "tough on crime". If a defendant is found guilty of a "capital" crime, send him to prison for life and let those who believe in his innocence, or his family, pay for his upkeep.
Governments lie. They manufacture evidence, intimidate witnesses, play on some people's "authority worship", and set-up innocent people who dare to stand against their edicts. Show me a perfect, just government that I would trust with the power of life and death, and I will show you a government that has no desire for revenge.
