Romans 13:1-7 — Should we resist ruthless regimes?

Still in today’s world, we have ruthless totalitarian/authoritarian dictatorships and regimes who are bent on killing innocent people and committing genocide (e.g., Assad of Syria, and Kim Jong-Il (through indirect starvation)).  There are also  governments that might not be as bad but are yet marginally ruthless. The bible has been used by some Christians, like Mennonites, to justify passive obedience, even in cases of ruthless dictatorships and regimes.  In Romans 13:1-7, Paul says this:

Romans 13: 1 Let every person  be subject to the governing authorities. For there is no authority except from God, and those that exist have been instituted by God. 2 Therefore whoever resists the authorities resists what God has appointed, and those who resist will incur judgment. 3 For rulers are not a terror to good conduct, but to bad. Would you have no fear of the one who is in authority? (ESV)

In what case should Christians refuse to submit to such governing authorities?

Should we as Christians never, in any case, rebel against the governing authorities?

Or would we be within ethical boundaries to physically and even violently resist abusive governments?