Here's the deal. If Syria gets more chemical / biological weapons, that's not good, but it's really not any different than before. They're just getting more of what they already have a lot of. They know they'll be dealt with if they use them, and they don't want to be dealt with. The Syrian government is a secular, Baathist organization that only exports jihad. They don't inhale.
As far as Iran and North Korea getting nuclear weapons, that's the real devil incarnate. We should do everything in our power to keep those weapons out of the hands of religious fanatics and insane cultists. I think we'll be in the position someday of looking back and saying 'Why didn't we act when we could have prevented all of this'.
And that is a reason to ignore them because? Nobody really cares if they want to kill their own people, it is precisely the exporting that is the problem.
And I agree, NK and Iran are really nasty and need to be taken out. Unless US military doctrine has advanced to the point where we could take them out with as few casualties as in Iraq, forget about it. People would never support it. We will just have a take a massive hit first.