I was the same.
(The following might be a bit Swissglish. My English goes weird when I have a migraine.)
I felt like that because Grace seems quite flippant about killing them all. Probably because her motivations are wrong. Her reasons for killing her father (and everyone else for that matter) aren't deep/real enough. Her drive seems off. She doesn't have a serial killers's psychology, and because of that, she could (should) have proper second thoughts before and after each kill. There's a lot of time between the killings where most folk would reconsider their actions and maybe stop. But she more or less keeps going in the same vein. She doesn't need the serial killer mind to kill, but she still needs the correct emotions and incentive, which to me she doesn't. Her rich dad left her mum to have another family etc and that's bad, but that happens to so many kids and they don't go out on a revenge spree. I didn't see Grace damaged in a way she is capable of killing someone. The story might work better if she simply wanted some sort of resolution with him, instead of killing him; i.e. make peace with him and 'find a father', an emotional connection. Or if she was a much more troubled human who has the capacity to kill in the first place.
Still, having said all that, I gave the book to my sister for Xmas.
The German translation.
Yeah, I agree. It is glamourized somewhat. Or just handled badly. Probably because it was written for the humour, not the character. I think Grace is too one sided, shallow even. I didn't see genuine feelings or depth of character. It felt it's played for laughs. Her soul doesn't hurt enough, nor is she complex. I think the author had an idea but didn't take it as far as she could have done. It felt frivolous, and written for effect; just thrown out for the fun of it. In a hurry.
I don't think it's a topic an author can have fun with when writing. Not fun in the fun sense. A story like that can have fun and humour, but not the way she did it. She could have used the subject to create a meaningful story. Alas, she didn't.
I was close to giving up too because of all the above but also because the MC's voice began to annoy me half way through. But you listened to it, so it may have been less annoying. I stuck with it mainly to see if it's a comp novel for mine.
Yeah, I hated it. It felt a 'simple' easy way to round it all off, and somewhat left field.
All in all, I thought it was an easy read, but fell short in many ways.
I read it because I'm writing a similar book (Mia - she too is looking for her dad to kill him. - I'm at the editing stage.) Mia has taken me 4 years so far because I want to get Mia (her character) just right. I wanted the depth, the damage, the pain of the past that drives her need to kill so she can have a future. Mia too has humour but the humour is dark and used as a tension breaker (that was the intension, anyway).
Humour and murder. Not an easy mix but an interesting one when it works.