I reckon the reason they were so opposed to Saddam was they thought he might be difficult to deal with when it came to oil, so they got rid of him in the hope of sticking a puppet-in-the-pocket-of-the-States type government in his place. Well yes, they well and truly fucked that one up for sure, but I reckon they might have the same view on Mugabe if there were oil in Zimbabwe.
And all that is assuming an oil-rich Zimbabwe didn't become a Chinese client state like Sudan. In fact, given the distrust (in many ways justified) that Zimbabwe has towards the west, I feel quite certain that Mugabe would forge closer ties with China (the main non-Western oil importer) rather than allow himself to be invaded / controlled by US/UK. A move that hasn't worked out too well for the people of Darfur.
Don't get me wrong, something should be done about Zimbabwe. Just as something should be done about every despot. But historically speaking, that's not what tends to happen when oil is discovered. Indeed, there's nothing western oil companies like more than a compliant dictator. It's populist leaders like Hugo Chavez who pose the greatest threat to their profits, not corrupt tyrants.
