I'd say it's more likely, in a situation like this, that one day she would seek him out, or his masked companion, in order to have revenge. And then he would have to kill Eris as well. What's there to understand?
Yeah...Wtf?
I specifically came to look for this fight, just because of the sheer proportion of the spells Rudeus tossed at Orsted in the novel. Out of all the things to skip, to skip the things that made this fight entertaining? Weak.