At this point in the play Macbeth is king, this quote is a juxtaposition, "Wicked" connotes being malevolent and heinous, meanwhile being a king connotes authority and being exceptional at what you do and being commendable, yet somehow Macbeth is King but fits more under being wicked. This ties into the theme of appearance vs reality, in appearance Macbeth is the King of Scotland, but in reality he is the false hero, a fraud, evil, malevolent. The witches also objectify him by describing him as "Something"