He replies, quite justly, that his will is working perfectly well. The will chose just what the man wanted, and he didn't want a cockroach. Jesus used another example besides that of fruit-bearing trees. If a man were to reach into a chest, he could only bring out what was already inside the chest. Different chests contain different things, and consequently, different things are brought out.
Different hearts contain different things, and consequently, different choices are made.
So when God saves a man, He does not give him a new will. As a result, the new Christian begins making new choices. No man is capable of making a choice contrary to the strongest desire of his heart. But what he finally does is what he wanted to do most, and he is therefore responsible for the choice. If the choice were not his strongest desire, he would not have chosen it.