From the Stanford University's MLK Research Institute, this is the full text of the speech King gave on April 4, 1967, a year to the day before his assassination. Read it and understand why he was killed.
His prophetic words warning of what would become of the US if it didn't halt it's vicious imperialist war on the Vietnamese people has come to pass. He was speaking a warning that those in power clearly never wanted spoken.
You can also hear him in his own words here