That was an honest answer. How else could your question be answered? You must know that no one knows why others do what they do what.
Certainly, when judging a crime, you should not presume you understand why the accused might have done what he is accused of doing. But in a general sense, when you posit that there is no such thing as good or evil, you are removing a particular basis for choosing one thing over another. Many people use good and evil as the basis for making their choices. You seem to want to replace that paradigm. I was asking what you want to replace it with.