"Abortion has nothing to do with the Las Vegas shooter."
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It has more to do with than the NRA does.
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Semi-automatic weapons and magazines have been around for years (decades?) prior to 1973. Even out-and-out machine guns were "legal" less than 100 years ago. But we didn't have Bonnie and Clyde taking machine guns into a theater and shooting 100 people just for the fun of it.
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Guns have always been available. Always. 
But Abortion has not always been legal. Not always. 
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Guns didn't change how we, as a country, and how we, as a society, think. But legalized abortion did just that. It caused a fundamental change/shift in how we, as a country, and how we, as a society, think about how - and if - we value human life. 
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We have had guns since before this nation became a nation. But we only had legalized abortion as the "Law of the Land" since 1973, a mere 44 years.
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The SCOTUS in 1973 re-taught some of us, including Mass Murderers, how to devalue human life....and then kill it.