He seems relatively benign today, in light of the absolute corruption of the Trump administration, but Rumsfeld was arrogant as well as cavalier about the idea of human rights. It was very costly to the US in nearly every way, human cost, money, and international standing. He was a 'neocon' so at least he believed in something as opposed to Trump and his circle which are very nearly nihilistic. The 'neocon' objective was ultimately to spread democracy to the middle east. In my eyes that isn't such a bad thing but the way they attempted it was arrogant and consequently disastrous. Errol Morris made an interesting documentary (as he always does) about Rumsfeld called 'The Uknown Known' which is streaming on Netflix.
By the way, I never bought into the notion that the invasion of Iraq had anything to do with oil. It was a foolhardy mission of idealism if you ask me.