"I mourn the loss of thousands of precious lives, but I will not rejoice in the death of one, not even an enemy. Returning hate for hate multiplies hate, adding deeper darkness to a night already devoid of stars. Darkness cannot drive out darkness: only light can do that. Hate cannot drive out hate: only love can do that." - Martin Luther King Jr.
A lot of things get lost in translation when a major death occurs and one of them happened to be a Dr. Martin Luther King quote after Osama Bin Laden's death.
"I mourn the loss of thousands of precious lives, but I will not rejoice in the death of one, not even an enemy." - Martin Luther King, Jr
A simple Google search reveals that the phrase cannot be traced back to King. Placing the phrase in quotation marks and searching it yields no confirming sources.
The Atlantic suggests the phrase was started by a 24-year-old Jessica Dovey, who is an English teacher in Japan. According to them, Dovey added the phrase to state her own opinion while adding a subsequent MLK Jr. quote, but accidentally linked the two.