You can't desire nothingness, you can only desire something. Augustine even argues that the suicide doesn't will nothingness or non-existence, although it might seem that he does. Rather, the suicide really desires peace. And peace exhibits the constancy that is the most conspicuous characteristic of Being. In other words, the suicide may think he wants non-existence, but in fact he wants existence of a higher kind, existence more like God's.
Every action is undertaken for the sake of something that the agent believes to be good.
Every good thing is good only because, and to the extent that, it resembles God. The initial problem about the theft of the pears is that it seems to provide a counterexample to the second assumption. There does not seem to be any good that Augustine was striving to get. He did not need the pears. He had better pears of his own.