Seems to me any country that refused to trade oil in US dollars or that has come up with a new way to trade in a different currency that affects the strength of the US dollar as a world currency. Is labelled a tyrant and has an immediate problem with civil war with fighting rebels backed by the US. Yes I agree that most of these leaders did some harsh and awful things to citizens but you cant tell me the US government doesn't have blood on its hands from all its past invasions. More countries trading in US dollars the better for the ever increasing Military budget to keep the world in check. Russia is next to fall off the perch and be slapped with the democracy book and to trade in dollars. But hopefully it doesn't look like good old Libya its doing so well now.
The U.S. Actually gets very little of its oil from Iraq; the total from all Middle Eastern countries, including Saudi Arabia, is about 15 percent so this question is fairly loaded. We actually went into Iraq to take out a tyrannical and unstable dictator who was threatening the rest of the world, and the same situation currently exists in North Korea - so maybe we should do it there too.
The Iraq War didn't really have much to do with oil. The reason why the US went to war against Iraq and keeps meddling in the Middle East's affairs in general is because of Israel. As that country is literally the only one in the region that's our ally and can keep the Arab ones in check, the US wants to do anything it can to eliminate any threats that could pose a danger to its closest ally.
While I'm sure they'd have liked to, the history of North Korea being evil to its own citizens is a lot more subtle than what Saddam used to do. When the war in Iraq was announced the general feeling around this country was "hell yeah, lets go romp them" and I think people would be more hesitant to back a war in North Korea.
I don't think the United States would have gone into North Korea to confiscate their oil and control the country. Reason being? North Korea has the 4th largest standing army in the world, don't listen to anyone, and they have very powerful nuclear capabilities. This isn't a country you just waltz into and take. Different story.