Speaking firmly, if occasionally stumbling over words, the 88-year-old president accused the United States of "rushing to suck oil from Iraq" when it invaded the country in 2003 on the erroneous grounds that it possessed weapons of mass destruction.
He said the UN Security Council had allowed itself to be "abused" last year by authorising "all necessary measures" - diplomatic code for military intervention - to protect civilians in Libya in a Nato operation that eventually toppled Gaddafi's government and led to his death at the hands of rebels.
Speaking with deliberate irony, Mugabe opened an address to the UN General Assembly by praising as "most glowing and most moving" a speech by US President Barack Obama on Tuesday in which he rued Stevens' death.
Stevens and three other Americans were killed during what Washington has called a "terrorist" attack on the US mission in Benghazi on 11 September. The assault forced the evacuation of US personnel from the eastern city that was the hub for the Libyan rebel movement.
"I am sure we were all moved, we all agree, that it was a tragic death indeed and we condemn it," said Mugabe, who has ruled Zimbabwe since independence from Britain in 1980 and is among Africa's longest-serving leaders.
"As we in spirit join the United States in condemning that death, shall the United States also join us in condemning that barbaric death of the head of state of Libya - Gaddafi? It was a loss, a great loss, to Africa, a tragic loss to Africa."
The Zimbabwean accused the United States and North Atlantic Treaty Organisation, the 28-member Western security alliance whose air strikes helped Libyan rebels defeat Gaddafi's forces, of acting under false pretences.
"The mission was strictly to protect civilians, but it turned out that there was a hunt, a brutal hunt, of Gaddafi and his family," Mugabe said. "In a very dishonest manner we saw ... Chapter 7 being used now as a weapon to rout a whole family."
Chapter 7 of the United Nations Charter allows the UN Security Council to authorise actions ranging from diplomatic and economic sanctions to military intervention.
"Bombs were ... thrown about in a callous manner and quite a good many civilians died. Was that the protection that they had sought under Chapter 7 of the Charter?
"So the death of Gaddafi must be seen in the same tragic manner as the death of Chris Stevens. We condemn both of them." Mugabe, a long-standing critic of the West, is himself widely criticised for turning what was once one of Africa's strongest economies into a basket case and has been accused of hanging on to power through vote-rigging.
Other speakers at the UN General Assembly on Wednesday - notably Bolivia's leftist president, Evo Morales, and Iranian President Mahmoud Ahmadinejad - also criticised the United States for what they see as economic and political bullying.
A US official had no immediate comment on Mugabe's remarks.
The Zimbabwean leader appeared to be in reasonable health despite questions about his wellbeing sparked by Zimbabwean media reports that he has travelled to Singapore eight times in the past year to seek medical attention.
He walked in an almost jaunty manner to and from the lectern in the General Assembly hall, where he read his speech from a written text.