I don't know of any lasting good Hitler did or see any rational comparison between the two circumstances, me thinks you merely look for absurdity, but feel free to disagree with the Biblically based reasoning as I presented it. Remember that I stated I would not name the holiday after MLK - so then it logically follows I don't attribute the end result to the man but only let the good shine through out of the darkness.
Why do you believe Columbus is worthy of honour when MLK is not? The discovery of America was doubtless a historically important event and to a certain extent necessary inasmuch as it eventually resulted in the existence of this country but it was accompanied by much evil including genocide, slavery, rape, and a host of other sins of which Columbus was not innocent. Was Columbus very historically important (just like MLK)? Yes, but I don't see why his actions merit celebration to the extent of having a national holiday.
I don't think every evil attributed to Columbus is true. Also as is evident by this thread, views of Martin Luther King are also changing, dependent on new research and findings.