I believe that Hans was actually a good guy all along, or at least had noble intentions.
Was there any actual foreboding that he was an evil cunt until that moment when he left Anna to die?
He's portrayed as a greedy, egoistic little rat that went to Arendelle with the intention to, at least, seduce or manipulate one of them and probably had already planned some murder.
But what if he was actually just there for diplomacy and had genuine feelings for Anna after meeting her? Could it be that all he did was for the good of Arendelle and its people once he realized that the princess is an absolute retard and the queen is a probably evil ice witch and monster?
He did took care of the emergency measures while Anna was hiking in the mountains. He didn't ask for this responsibility and was probably a bit worried by Anna's weird demand. She was gone for like more than a day, right? He must have been in contact with citizens and officials during that time, most likely started to care for them. He must have seen that Anna is very stupid too to leave him, a stranger, in charge of her kingdom in a time of utmost crisis.
Then he went into the ice castle with the party. When he got in, he witnessed Elsa about to gruesomely murder two people. Yet he stopped her for doing so, a thing that she could very well have done in self-defense and in the heat of the moment and that she would have never forgiven herself.
But he also saved Elsa's life from the crossbow guys. If his plan all along was to have Elsa die in order to marry her stupid sister and become king, all he had to do was not intervene. Why did he save her and had her brought back to Arendelle just to kill her with his sword in front of everybody a couple of hours later? I think he really wanted to help her and save her up to that point, thinking that she would remove winter if he brought her back.