They called him Killer because his name was Killebrew but his personality was the exact opposite. There are hundreds of stories out there about how kind and gentle and approachable Killer was. But at the plate the man lived up to the name. As the incomparable Joe Posnanski points out, he was inhumanly strong and hit home runs at a pace greater than Mickey Mantle, Ted Williams, and Sammy Sosa. He was born to rake.
In his career he hit 573 home runs, including eight 40 homer seasons (and one 39 homer season), which is good for 11th all time. Though the list is now polluted with steroid users, at the time he retired he was
There has always been a rumor that Killer was the model for the MLB logo, though it is not entirely clear. You can read up about it here. Killebrew always maintained that it was him, and the man that supposedly designed it maintained that was just a composite of a lot of different batters. In any case, he is an iconic figure in baseball, the face of Minnesota Twins, and, in my opinion, one of the mythical "inner circle" Hall of Famers.
Dale Murphy was a pretty great centerfielder and has a good case for the Hall of Fame, Jacoby Ellsbury is a Red Sox which automatically makes him capital-G Great, Jeff Kent is one of the great offensive second basemen of all time, Bryce Harper is quickly gaining legend as perhaps the greatest prospect ever, and when all is said and done, Roy Halladay may end up taking the title of greatest Mormon baseball player of all time from Killebrew and leave Killer just as the greatest Mormon hitter of all time, but for now Harmon Killebrew stands alone, and baseball and Mormons have lost a great one.