I'm really not sure what "nastiness" you imagine "King Charles’ camp" put out so it's a bit difficult to object to the "callousness" of something that doesn't exist, but from what I've seen of the excerpts from the book that have been serialised and in the interviews he's been doing, Charles Spencer does not paint a particularly pretty picture of Diana at all. For a start, he reveals to the world that Mohamed Al-Fayed allegedly sexually assaulted her - an incredibly private thing that she is unlikely to have wanted the world to know. He also "confirms" the story that Diana told someone else in complete confidence, never imagining that it would be broadcast to the world, that Diana pushed her stepmother down the stairs and enjoyed it.
Lol at the idea that I'm dictating what Charles Spencer writes. I'm just commenting on what the title reveals about him.
As for him "being there", he apparently wasn't even talking to Diana in the last period of her life. He also says that at various times Diana fell out with every single member of her family, even if she did eventually make up with them. Yet another nasty thing to say about her and, even if it were true it's a private matter and it's not his business to reveal anything about Diana's relationship with anyone but himself.
https://people.com/princess-diana-brother-charles-spencer-on-conspiracy-theories-her-neurodivergence-and-more-exclusive-12130540
He diagonses Diana as having had ADHD, even though he's not a doctor, and by saying he believes she had ADHD he's basically saying that she had mental health issues and behaved oddly and caused problems.
Spencer never even told his other two sisters that he was writing a book about their sister - and he admits they were hurt by this. In fact, the only people who knew were his current wife and his publisher. So, this book is really only his very personal interpretation of events from 30-40 years ago and earlier. He can't have done much research for it, he obviously never did interviews with his sisters or any other people who knew Diana to get more stories and memories about her and to corrobarate things. It's basically all plucked from his memory (even if he does admit that some of the things he believes now are not necessarily what he believed at the time) and I presume some of Diana's personal papers that he claimed for himself as the "head of the family" and which really should be in the possession of her sons.
In short, from what's been revealed about it so far, Spencer's book does not portray Diana in a particularly nice way at all and it's full of Diana's private thoughts and experiences (or what Charles Spencer says were Diana's private thoughts and experiences) that Charles Spencer does not have the right to reveal.
Edit: typo