So what was Malenia's purpose?

After DLC I felt that Malenia was very out of history, she fought her entire life against the rot, her blind master taught her also to fight fluidly to avoid rot, Miquella was also looking for a way to heal her

But then she leaves everything for a meaningless struggle against Radahn and explode the scarlet rot in Caelid, then Miquella goes to the shadow land and leaves everything too, the Haligtree, the healing of rot, the needle, his own sister, everything

I think everyone agrees that this is a very big change in the form that Malenia and Miquella acted and in their personalities, what happened to occur this change? When did it happen? And does Radahn have any role in this change?