Why does Benny Morris argue that "there are circumstances in history which justify ethnic cleansing"?
Basically, what Benny Morris argues is that, when you have to choose between ethnic cleansing and a situation where your own nation ("your kind" of people) will be destroyed, you have to engage in ethnic cleansing. He is essentially saying that there are times when a people might have to engage in ethnic cleansing as a sort of self-defense.
Morris says this in regard to the creation of Israel in 1948 and the way in which the Palestinians were largely forced out of Israel. He acknowledges the damage done to them and admits that it was ethnic cleansing. However, he argues that it had to be done because there was no way to safeguard the Jewish people without doing so.
Overall, then, Morris is saying that ethnic cleansing is bad, but that it is sometimes necessary to protect your own kind of people from annihilation.