IS, also referred to as ISIL and ISIS, released a video Sunday that showed Kassig's decapitated head, making him the third known American executed by the Islamic State in recent months. Videos were released in August and September that showed the execution of journalists James Foley and Steven Sotloff.
"Abdul-Rahman was taken from us in an act of pure evil by a terrorist group that the world rightly associates with inhumanity," the president said Sunday. "Like Jim Foley and Steven Sotloff before him, his life and deeds stand in stark contrast to everything that ISIL represents."
Kassig, a former U.S. Army medic, established Special Emergency Response and Assistance, a non-governmental organization, to assist the wounded in Syria, treating both civilians and fighters alike. He was abducted in 2013 while delivering medical supplies. During his captivity, Kassig converted to Islam and took the name Abdul Rahman.
Obama contrasted Kassig's work with the terror group's "act of pure evil."
"While ISIL revels in the slaughter of innocents, including Muslims, and is bent only on sowing death and destruction, Abdul-Rahman was a humanitarian who worked to save the lives of Syrians injured and dispossessed by the Syrian conflict.
"While ISIL exploits the tragedy in Syria to advance their own selfish aims, Abdul-Rahman was so moved by the anguish and suffering of Syrian civilians that he traveled to Lebanon to work in a hospital treating refugees. Later, he established an aid group, SERA, to provide assistance to Syrian refugees and displaced persons in Lebanon and Syria.
"These were the selfless acts of an individual who cared deeply about the plight of the Syrian people.
"... the indomitable spirit of goodness and perseverance that burned so brightly in Abdul-Rahman Kassig, and which binds humanity together, ultimately is the light that will prevail over the darkness of ISIL."
The president extended his condolences to Kassig's friends and family.