Sheryl would have never left since she cares a lot for the ppl of Frontier and all her singing is for them now. Alto understood this, and respected her decision, but paid the price in return (stayed)...
Perhaps Alto actually thinks that Leon is an usurper, but he just loves/cares for Sheryl so much now that he was willing to sacrify justice to protect her... Maybe that's why he screamed in the end too...