[cmccabe]

why not just kill them right away?

Because evidence might come up later that proves them innocent. Because killing someone deprives them of all of the rest of their life, whereas life imprisonment offers them some fraction of it.

I actually do think there are some people we should just hang-- after due process, of course-- but definitely not everyone who has a long sentence.

Also, a "life" sentence usually just means 20 years in the US, for whatever reason.