This is because 'to hit' can of course mean "to punch someone" in English, and then 'ler' is the present tense of the Norwegian word for "to laugh". Put these words together and it made some amount of sense to me to imagine old "hit-laughs" as someone who hits people and then laughs at their pain, in other words a bully, a sadist, or otherwise a cruel villain.
Who was the previous Hitler? Who will be the next one?
(apparently it was pretty inconsistent before Hitler, the stand-in for "great evil man". Napoleon and Judas were pretty popular English ones, but nothing reached Hiterian levels of reputation)