belboid
Exasperated, not angry.
I think it's the one-offness of TKAMB that gives rise to the rumours. As noted, HL did a lot of the research for in Cold Blood, and it is sometimes claimed that actually she actually wrote that and so Mockingbird was his payback.I think - well, if it's true it's cynical... but if it's true, she's probably past knowing about it. I will doubtlessly read it myself, and i'll not fret too much.
Regarding the Capote thing - firstly it's got patriarchy written all over it: great book? Are we sure it wasn't a man who wrote it?
Secondly, it would be very odd if Capote saved his best fiction writing for publication under another's name. I've not read all of his work, but I've read a fair bit and he's doesn't really demonstrate the sustained skill that we see in TKAM. Which is fine - because in his finest moments he's an utter joy.
All put to bed now tho, as a letter from Capote from a year before the book came out clearly shows Lee was the author, even if TC made some suggestions and editorial amendments.