why does macbeth thinks his supernatural soliciting cvannot be ill ,cannot be good ?

Macbeth is aware that witches are considered evil. Banquo reminds Macbeth that the witches will, The instruments of darkness tell us truths, Win us with honest trifles, to betray 's. In deepest consequence. He thinks his news is good but at the same time, it is tainted coming from witches.