Why don't we have the death penalty for most crimes.?

I think we should have the death penalty to stop crime and lower the taxes that keep inmates locked up living the good life. My proposal is that if you commit a crime, no matter what crime and you are found guilty you get the death penalty which will be executed within 10 days of your verdict. The only thing that won't be any death penalty for is a speeding ticket under 20 mph. You might think this is insane, but if we started to do this, our citizens would start to obey and will always try to do the right thing. The ones that do wrong are problematic and should die and be made an example of. Sounds sooo good right, but why don't we do it.