I am having a hard time with the ending to the series. Mac is now in love with a man who fed humans to his son. He probably eats humans himself, although Mac is carefully not asking about that.
Barrons is an implacable enemy. You are with him or you are against him. He is an animal and he sees that as a simpler, more straightforward thing to be. Mac sees him as having a deep sense of honor. To me it just sounds like thinking in terms of me and mine.
Fiona betrays Barrons and he throws her to the dogs. He lets Mac off for siding with Darroc because he knows she wasn't betraying him. Why does he let her off for not keeping her promise about his son? Love? Didn't he love Fiona in his way? Because he knows she needs to save the earth? He says many times that he doesn't care about saving our world and would kill to get the Book/save his son. So why is it okay now?
And if Mac does something he considers to be against him in the future, will he let his friends kill her?
And what does it mean to love someone who has thousands of years of bad deeds behind him? He's killed for money. He's raped for fun. He killed one of his best friend's lovers. He brought a woman to his secret lair and made love to her knowing that his friends would then kill her. He tells Mac again and again not to romanticize him and believe good things about him (he told Fiona the same thing). Love is not going to change this bad guy.
Maybe part of the problem here is the tension between romance novels and fantasy/action stories. In romance novels, the hero is bad or wild, but true love reforms him. What makes the Fever series great is that it is not just a romance, it is a big story about good and evil and danger. For the action part of the story, Barrons has to be truly bad. I think he goes too far to be a romance hero at the end. I wanted HEA for Mac, but now I wish the series had ended in a way that was true to the characters. She saves the world and finds happiness in rebuilding it with a good man somewhere in her future.