Of course Harry was being irrational and stupid in Changes. The entire story is him going completely off the rails to save someone that he only really cares about on a conceptual level and throwing the people he actually loves under a bus out of sheer selfishness, and then killing himself because his martyr-complex-self is more comfortable with him dying 'nobly' then owning up to the consequences of his own asshattery.
Doesn't change the fact that Susan was not rational in hiding Maggie from her father, and is apparently even more allergic to rational thought then Dresden himself.
Disagree with your interpretations. I agree that Harry was irrational all through Changes, but it was because he cares, not because he's got some noble death fixation. Think: Harry has been an orphan since he was what, 6? His adoptive father was abusive, twisted, and evil, and then was killed by Harry when he tried to mind control him (and succeeded in mind controlling Harry's first gf). His first job as a private detective was with a guy who searches for missing children he rarely finds and apparently can only afford to live in his office in a run down, bad part of town on the border of gang territory. With Harry's skills, he could easily have built a much easier life for himself, but he doesn't, he continues to do what he thinks is right. He is constantly describing how happy Michael's kids are in the books. Also, remember when he got his hand fried? Harry has a fixation on Family, and another on saving and protecting children from all the horrors of the world. He's going on , trucking along, witnessing all the horrors of the world (not least of which is what the Red Court does, especially when it targets young people) and burning them down whenever possible
for people he has never even met before, and then, suddenly, he finds out out of nowhere that not only does he have a daughter, but the red court has her and are probably planning to do something horrible to her. Changes is nothing but Harry dialing up his usual behavior to save
his own daughter, his
family, you know, that thing he always, always wished he had for his whole life. The situation has changed, he now has something personal to fight for, and he goes completely off the rails trying to do it. Make no mistake, children of your own change everything, whether you've met them before or not. The only people who wouldn't be affected are those who are married to cold, hard, logic (or people trying to apply cold economics to Harry Dresden), and when has that ever described Harry Dresden?
As for his death, he already knew Susan had fled from Chicago, apparently to get away from him (because Harry Dresden, deep inside, thinks he alone is the source of all his friends' woes). She has a kid,
his kid, hides it from him to protect the girl
because it's too dangerous for Harry to be around her. Then, things happen and he breaks his back
and is helpless to save his daughter. At this moment of deepest darkness, his absolute lowest point, he needs a miracle. Instead, he gets the seven words, and that's it, that's all it takes to focus his wild actions toward a single result: saving his daughter. He calls kincaid and accepts Mab's deal because it is literally the only way he can save the little girl. He is clearly terrified of Mab, and just as clearly terrified of what she'll do to him. He sees her as cruel and vicious and uncaring, and he doesn't want anything to do with that. He knows he would be a much more powerful and dangerous winter knight that Slate ever was, he fully expects to turn him Lloyd Slate Mk 2, and he doesn't want that for anyone. But, most of all, he doesn't want the fae going after his daughter to get at him.
Harry Dresden's suicide, to him, was the best he could hope to get out of many, many terrible options. Not only for him or his friends, but most of all for his daughter, his
family.
It was a terrible decision, but based on what Harry knew and
thought he knew at the time, not to mention the seven words driving their way through his skull, it was the best he could come up with, for everyone.
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Also, agreed Susan was being an idiot with how she handled the whole thing as well. But, something to remember is that one can be extremely intelligent and still make terrible decisions. It sounds like Susan's INT is high, but her WIS is definitely not. She wanted to be a mother to Maggie and wasn't able to convince herself to leave her somewhere that actually WAS safe and out of reach, and instead precipitated the whole thing by bringing her with her.
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[edit] Also, what Maz said.