if a traditional GOP candidate wins, millions will die in the wars they start - guaranteed. Listen to the likes of Santorum, he's virtually promised to deliberately start a war with Iran no matter what - and Iran, unlike Iraq, will be no small feat to conquer. It's as if he wants to restart the Cold War, and it would - it would force Russia and China to support Iran for their own self-preservation.
If the libertarians take over, some people will die from neglect, but not anywhere near the body count a conservative would produce. And the problems caused by them are easier to mitigate (e.g. we could set up SS/Medicare/Medicaid equivalents on the state level and leave the federal government out of it).
Perhaps the most dangerous part of getting a conservative is that when we do go to war under them (and they leave no other possibility on the table) almost all the Democrats will be conned and intimidated into supporting another insane war, just as with Iraq.
I fear a conservative far more than a libertarian. We could survive Ron Paul, especially since Congress would limit his ability to act. We could not survive a conservative who now has the unlimited power to start wars (thanks to Obama for the destruction of the War Powers Act over Libya) and assassinate political opponents (thanks to Obama signing NDAA).
While a libertarian would eschew those powers, a conservative would use them (frothing at the mouth all the while, probably), and that is what really scares me. It makes me want to look up backup plans like fleeing to Canada.