In the kite runner, assef tells amir, “afghanistan is like a beautiful mansion littered with garbage, and someone has to take out the garbage.” in assef’s mind, this justifies

I believe that in Assef's mind, this justifies ethnic cleansing.
He says that there is a lot of garbage in his beautiful Afghanistan - garbage of course meaning people who deserve to be killed, according to his own opinion. He is practically committing a genocide - going after each of these people he considers garbage and killing them, thus justifying his own belief they are not worthy of being alive.