I really enjoyed this. My crackpot theory that makes no sense (which is my general method because Agatha Christie was an evil genius so I always go for the most out-there possibilities) is that Leonard and Romaine are in on it all together.
Or the cat dunnit. Yeah, I'm not really good at working these things out.
I hope it isn't compared too much to And Then There Were None though. That was essentially an early slasher, this is a courtroom drama so of course it's slower paced etc.