Generally, I believe that people should stay and work out their differences after losing an election. But, recently I've been wrestling with when circumstances fall beyond repair and it would be reasonable to leave. In hindsight, it was reasonable for people to emigrate when the Nazis came to power in 1933 especially if they weren't in a position to affect change. But, where's the line? How dire does a situation need to be for leaving to be justifiable?
Clearly, the bar for leaving one's country should be high. Differences would have to be fundamental. For example, people would need to believe that the system itself no longer allows for a meaningful democracy. If a country criminalized difference in opinion and intentionally abrogated citizen's human rights, I'd leave. People don't owe unconditional loyalty to a system that may ultimately betray them.