Is it ever okay to assassinate foreign leaders?

It's an interesting question in international relations, the use of assassination as a tool of policy. In the US, the government use of assassination is illegal, but it's questionable whether that rule still stands in practice after September 11 (and eight years of Bush-Cheney rule). But the larger question is: Should it be illegal?

When I read about people like Mugabe in Zimbabwe, I'm hard pressed to find an argument against just killing the guy. Putting geo-political costs and benefits aside (e.g., will the situation truly improve if Mugabe is disposed of (think Iraq post-Saddam, not necessarily better)), the question I'd like us to consider is, morally it is acceptable to simply have Mugabe killed?

Yes, it's a stark question, but I think it's an important one. Look at South Africa. Mbeke spent years denying the origins, scope and treatment of AIDS, and he successful doomed hundreds of thousands of his own citizens with his denialism. I've long believed that Mbeki is no better than any other leader overseeing and orchestrating a genocide. And I doubt many would have a problem taking out a foreign leader overseeing a genocide. So would it be appropriate to apply the same logic to a Mbeki or a Mugabe?

And then, after we've debated the moral question, it isn't a bad idea to have the practical debate as well as to whether replacing a foreign leader would make any difference. I believe the US, and others, have tried in the past to install our own guy in a number of countries (see Afghanistan and Iraq), and it's not clear it works. Would getting rid of Mugabe, or having gotten rid of Mbeki, truly have improved the situation in South Africa or Zimbabwe?