Half a year ago, I had a major complaint about the script of Frozen, centered on the complete lack of buildup to Hans suddenly turning on Anna. Not a hint throughout the film, and in fact, he makes a rather selfless gesture not long before. It's just bad myth-making, bad narrative. One might not see a turn to evil the first time, but on repeated viewings there should be hints along the way.
Apparently there were hints, and more, but they never made it to the script because of other plot considerations. There's a fascinating explanation of those changes in this Weekly Standard article by Jonathan V. Last. Short version; Hans was not originally evil in the script, but when the plot changed, someone had to set the last rescue scene on the ice in motion, and nothing else was ready to hand. So Prince Hans, contrary to his good nature so carefully built up in the first 90% of the movie, had to be called into service as the villain, because there was no one else there.
Told ya it didn't make sense.