Why is Prince Hans so helpful towards Elsa?

Prince Hans' main motive is to become King of Arendelle. To do that, two things must happen first:

1. He has to marry Princess Anna ('As heir, Elsa was preferable, of course, but no one was getting anywhere with her.' So he turned to Anna).

2. Queen Elsa has to die.

After Elsa flees Arendelle and Anna follows her Hans' plans are put on hold. Both Elsa and Anna are now out of his reach. So he has to play the Good Guy for the time being and bide his time. When Anna's horse returns alone his plans are now in jeopardy. He needs Anna alive in order to marry her. Queue the rescue party. Eventually the party stumbles on Elsa, but no Anna in sight. He can't kill Elsa yet or have anyone harm her since it would jeopardize his chance to marry Anna. So now he has to keep Elsa alive and safe until he can find her sister, which means he had to rescue her from the assassins and then bring her back safely to Arendelle. Although he does keep her under lock and key.

When Anna finally arrives in Arendelle, all the pieces in Hans' master plan are in place. He fools everyone into thinking that Anna is dead and has shared marriage vows with him and orders the execution of Elsa. Had things gone his way he would've become King Hans of Arendelle.

Princess Anna wouldn't likely have agreed to marry Hans if the detail he was in charge of resulted in Elsa's death, even though he himself didn't cause it. When Anna requests a "true love's kiss" from Hans and, knowing he couldn't deliver, his undercover plot couldn't continue his true intentions are outed. The death after unwitnessed marriage vows thing was a little silly, but technically Princess Anna did leave him in charge when she left. I admit that it is rather thin.

Everything Prince Hans has done up to that point was to court Anna.

Helping her save her sister is his golden ticket to securing Anna's favour, which is important if he hopes to marry Anna. This is a crucial part of to his plan to seize control of the kingdom, and he passes up no opportunity to keep making a good impression on her... up to the point where he learns that Anna will die of the wound Elsa inflicted upon her.

Once Hans believes himself capable of getting rid of both the princesses – and without any risk to his public image – he no longer sees the need to maintain his facade in front of Anna, and is free to reveal his true colours.

The reason Hans did this was to cement his trust as a helper or Hero of Arendelle by being one of the few to stick up for Elsa, this way, once he is married to Anna and Elsa dies of an accident he will not bear any blame as he tried to save her before. It was also important for Winter to be reversed back into summer but once he became convinced that couldn't happen, he decided to kill Elsa early and paint her as a monster who killed her own sister, thus making himself out to be a Hero which also meant his marriage to Anna was no longer necessary.

I think that prince Hans wanted to be become the king of Arendlle and he has to marry Princess Anna kill Elsa because Elsa became queen as there was no man in the family and Elsa was elder so if Elsa dies Hans will become the king as husband of Anna. He has to maintain his image so he tells everyone that he had married Anna and Anna was assassinated by else, this way everybody agrees to kill Elsa because in their sight Elsa was a cruel and selfish person. After Elsa nor Anna exists he could be crowned as King but soon op good he was unsuccessful in his evil plans.

If you look at Hans just before he leaps he looks up quickly. He saw the ice chandelier and aims the crossbow at it.

Elsa was only defending herself against two men who were trying to kill her. She would NOT be considered an irredeemable monster even if she did kill the men. They were the ones who invaded her palace with crossbows and started shooting. She begged them to stop, but they kept coming. She had to fight to survive. They were the ones who tried to kill her and refused to stop even after Hans intervened. Elsa stopped.

Elsa would be very remorseful and feel the burden of her guilt because of what happened. She wasn't out there to kill people, she only wanted to defend herself. In my opinion, she would be morally justified in self defense.

As for Hans, he wanted Elsa to LOOK like a monster to the Arendelle guards. He then aimed the crossbow at the chandelier to attempt to kill her via the chandelier.

Well, first I don't understand why everybody thinks he is a bad guy. You will say he tried to kill Elsa and Anna but isn't it true that because of Elsa the whole population was going to be extinct. Second, it was Elsa who almost killed Anna twice. And about killing Elsa well she was a threat to the whole world so the choice is one Disney princess versus all the citizens of Arrendale so he made the right choice. So if Elsa has a pure heart so does Hans and if Hans is evil so is Elsa now decide.