Christians should stand up against Assad’s genocidal socialist tyranny. If they stand with Assad then they bring shame to Christians everywhere and should beg God’s forgiveness.
Since the realistic alternative to Assad is an Islamist regime, I fail to see the benefits in removing him from power. If you remove secular dictators from power in the Middle East, they're not replaced by western-style constitutional democracies, but by mullahs and imams.
Incidentally, the Christians in Syria aren't being killed off by Assad. They're being killed off or driven out by the rebels.