Excuse me,where do you get off saying that Allah SWT has the ability to do zulm?that dhulm is within the power of Allah Subhanahu wa Ta'ala and he has made it haraam upon Himself.
None of the maturidiyyah or the ashariyyah even consider it applicable to Allah.
It has nothing to do with something being wajib upon Allah SWT.
That hadith you posted has already been explained by the ulema,it has nothing to do with Allah SWT being able to do zulm.
Saying Allah can do zulm is like asking if a square can become a triangle and remain a square,the question or idea is simply meaningless,a meaningless bunch of words.
I'm surprised you said this muawiya,I certainty hope you don't believe it.