Killian's motivation was to control both sides of the War on Terror - supply terror, then supply weapons to fight the terrorists. Extremis was able to do both. Subjects that couldn't regulate blew up. "The Mandarin" claims it as a terrorist act deliberately conducted to "teach America". Killian supplies Extremis subjects that can
regulate to the U.S. military, either as soldiers or as mercenaries. Either way, he wins, and proves to himself that he didn't actually need Stark in 1999.