There is bad tyranny, there is worse tyranny, and then there is the worst tyranny.
When idiot Jimmy Carter destabilized the Shah of Iran, and thereby triggered the revolution that brought the Ayatollah Khomeini into power, we went from "bad tyranny" to the "worst tyranny."
The religious nut jobs running Iran are the absolute worst possible people to be in charge of that nation if you are concerned about what is best for the United States.
The Shah's secret police killed 3,000 people over a thirty year period. The Ayatollah killed 10,000 in one year. Now Iran is making nuclear missiles with which the religious kook bags will kill millions.
By the same token, Assad is bad, but do you know what would be worse? More religious nutbags taking over Syria. At this point, I would rather just put up with Assad.
As bad as the Mullahs are in Iran (and they are bad) they seem sane compared to the jihadis out of the Sunni Gulf be they ISIS or al-Qaeda or the Muslim Brotherhood.