You don't get it. The Left may not like incidents like Vegas. Nobody in their right mind would except for the extreme Leftists (of whom I sure exist). The problem is that Leftist thinking and policies and their PC pandering enables these types of events.
And you're definitely on the slippery slope. Think more about implementations and abuses as you try to constrain who is qualified to retain rights.
Your "slippery slope" idea is BS. Think about it! For eight years we heard from the NRA crowd that Obama is coming for your guns. Did you notice how that worked out? Obama signed more pro-gun legislation than anti. Check it out.