The magic in Frozen is spread by touch. And the original story’s change of Kai is immediate, changing him to a cruel and aggressive man instantaneously.
I’d just have the story be that Hans was really just interested initially in royal power, and didn’t have deep love for Anna, but the darker desires to kill to get that far were far deeper emotions he normally wasn’t prepared to act upon. Hans up to the point the magic hits sees saving the kingdom as the best way to secure his future, but murder only springs up as THE option when he’s cursed. The song before Fixer Upper was all about ‘getting the fiance out of the way’ and the trolls beyond the elder seem more impulsive, maybe they fuddled with the spell a bit to be transmissive.
So whatever spell was affecting Anna’s heart, still creeps in Hans now, and to fix THAT, Elsa has to give Hans an act of true love. And again, to mirror Frozen 1, I don’t see that as a kiss. I see that, as forgiving Hans. The other big element of the Snow Queen was allegories of sin and assumptions of irredeemability and hopelessness, but the Christian elements of salvation and hope are also present. And as they say, God’s Love is the forgiveness and sacrifice to replace sin. To perform an act of forgiveness, at the cost of something important, would be Elsa’s act of True Love for Hans.
So I see Frozen 2 as Elsa getting to meet the OG snow queen, all snow bees and everything. And part of OG’s thing is taking a darker path than Elsa to make her powers, and also explaining part of the magic’s origin. That way, Hans’ part can be that OG wants to use him as a magical conduit to complete the troll mirror to find and collect all those affected by the magical source. Of course this will kill Hans, but in turn the magically afflicted will join OG and Elsa far north to grow their talents away from the world. And Elsa’s only way to stop this is to take that potential and Let it Go