To ask whether Macbeth is or is not a moral play is a complicated question. Shakespeare wrote plays, not essays and he doesn’t tell us how he intends Macbeth to be read. There are obviously actions such as murder, betrayal, witchcraft, and war to which most readers of the period would have applied moral judgements, but that is not identical to claiming that the play has a specific moral.
There are two themes you could address in an essay arguing for this as a straighforwardly moral play. One is the supernatural. Many of the supernatural elements of the play (witchcraft, apparitions, the spot on Lady Macbeth`s hands) would have been seen in the period as indicating the work of the Devil.
Another theme might be the degree to which power corrupts, i.e. that to obtain power and riches people will act in immoral ways.