Alina essentially realizes that the Darkling might have a point: perhaps it is better to turn the Fold into a weapon that could work
for Ravka, rather than getting rid of it. However, she also knows now that the Darkling is greedy beyond belief, so even if he did the right thing initially, he’d soon wield too much power and more innocent people would die. Further, if Alina did go along with the Darkling’s plan, she’d be powerless (assuming Baghra told her the truth). So, nobody would be able to challenge the Darkling’s power again.