"I still find these people despicable."
Because taking drugs and shoplifting are enormous threats to society and the public welfare. To me, it doesn't make sense that someone gets 10 years for shoplifting some gadgets (total value, £1000? also virtually a victimless crime) when someone can get 0 years for stealing millions - 'sup fraudulent accountants and inside traders - from a huge cross-section of the public.
That aside, bear in mind that few of the people who commit major crimes are major criminals. These hardened criminals are imprisoned for numerous offenses at once - if indeed they're caught. Most big crimes are crimes of passion, so they could be committed by virtually anyone, although for obvious reason the lower classes are more likely candidates. I wouldn't call a woman who, after years of an abusive marriage, kills her husband for sleeping with another woman as big a threat to society as a guy who kills someone, takes their wallet, and dumps the body in a river. Even in cases where people might appear heartless (gang killings), it's common for them to be crimes of passion ("That fucker shot my best friend! Let's get him!") rather than anything that's been heavily premeditated. Thus, characterising all criminals who commit the big crimes as 'dead-enders' is madness.
Not that I don't think that certain types of criminal should be considered for execution (though there are moral and practical grounds that, I'd argue, make execution unethical; I'm undecided on the issue but I think that the idiocy of the masses probably makes it a bad idea), but saying that this applies to everyone with a life sentence is bloodthirsty and insane. Life sentences aren't equal.