How is it clear? No charges, no trial, no record. All we know is that they were shot in the head at close range with their hands tied behind their backs and then dumped into a grave next to where the Serbs were hiding other Albanian bodies they had brought in from Kosovo. It was murder as even the Serbian authorities now say themselves. Do you have other information?
As to motive, you can make the case that it was just one more cold-blooded racist elimination of Albanian unter-menschen, but it may very well have been that the brothers observed the body disposal effort and had to be eliminated as witnesses to yet another Milosevic sponsored war crime.
Who just happened to be armed and have the misfortune to enter Serbia along a route being used by terrorists.
Hey, if you want to side with the terrorists, that is your choice. Some Americans choose to mourn 911, others choose to weep for the 19 who caused it.