why is voting for a 3rd party a bad thing?
Gonna take a stab at this one: I think that people get upset when they see people who are in the same political alignment as them vote for a 3rd party because they see it as wasted energy that could have been employed in favor of the candidate that had a much better chance at winning.
In other words, in an ideal world, there would have been candidates from third parties that would be
very likely to reach 270, and the same person that would have gotten mad at the other before for voting for a third party would end up jumping ship and voting for the same candidate that their friend had voted for.
However, there hasn't been a viable third party candidate in decades, so when people do vote for 3rd party candidates, particularly in swing states where the margin of victory is very small, the third party candidates only serve to hurt the people of the political alignment that they share. People like Ralph Nader and Ross Perot are blamed for liberal and conservative losses, respectively, because they drained votes away from the primary standard bearers.
tl;dr: third party candidates hurt viable candidates in swing states and cause more harm than good by taking away precious few votes that could have made the difference between victory and defeat
