Specifically, he was the first person to implement a program to assassinate using drones, when he convinced the government to arm surveillance drones to try to kill Bin Laden.
Even Clarke thinks that Obama administration's drone program has gone way too far:
They were targeting people in so-called signature strikes, when a place look like a terrorist camp. And they were able, after looking at that place for days on end, to satisfy themselves that it was a terrorist camp. Then they attacked that camp without knowing, frankly, the names of the people who were there. The result was, collateral damage. We don't know how much. There are widely varying estimates of the number of innocent people who have been killed in each of these cases. But, we do know that innocent people were killed. As recently as the attack in Yemen at the end of last year that blew up a wedding.