It's socialism versus individualism.
It's better that society deal with crime - even at the cost of defensless innocents dying at the hands of criminals, than have individuals "take matters into their own hands."
deaths due to self-defense have tripled since the law was enacted
That's a bad thing? Every death due to self defense is one less rape, homocide or robbery of an inocent person. But to the people sponsoring this amendment, it's better to live with a very large population of criminals than have a very small incidence of "vigilantes".