Darko Brevic was a soldier alongside Niko Bellic and Florian Cravic and several other men in the Bosnian War. During this time, he had a drug addiction and sold out the squad for $1000 to help pay for his drugs. Only he, Niko and Florian escaped from the ambush safely, and Niko then hunted both him and Florian afterwards to find out and get his revenge on the one who did it.
To do this, he moved to Liberty City, the place he had discovered Florian to be living in. He then eliminated FLorian from his search as he discovered he was innocent, and focused solely on Darko.
Niko worked for a contact at the U.L. Paper, in return for the contact finding Darko for him. The contact is able to locate him, and drops him off at Francis International Airport for Niko to do what he wants with him. However, Niko learns from Darko that he is guilt-ridden and drug addicted, and he tells Niko he would be doing him a favour by killing him.
The player must then decide whether or not to kill Darko or not. If you choose to let him live, Niko feels better for it and he sees that letting him live was the worst punishment for him, and as you walk away Darko will get up and run off, never to be seen again. If you kill him, Niko will go away not feeling any better.