"Oh, yes," said Lord Peter, "but most of us have such dozens of motives for murderin' all sorts of inoffensive people. There's lots of people I'd like to murder, wouldn't you?"This is of interest because this is precisely the opposite of my theory of motives, although in effect it is the same in that it discounts motive as a detection tool. Here's my protagonist Schneemann after summing up the possible motives for each suspect:
"Heaps," said Lady Swaffham. "There's that dreadful - perhaps I'd better not say, though, for fear you should remember it later on."
"Well, I wouldn't if I were you," said Peter amiably. "You never know. It'd be beastly awkward if the person died suddenly tomorrow."
Schneemann waved it away. “As I said it was a rumour. I would not usually mention it, but if true it might provide a motive. Still, although all of these are plausible, they seem a little thin don’t you think? Nothing that would insist that the man be killed. Not to my mind.”It is my contention that in most (fictional) cases the motive for murder is inadequate. In general people provoked, threatened or injured in that way accept the circumstances and get on with their lives. Indeed it is this very failure to justify their crimes in this way that makes us condemn the criminal. A hungry person who steals to eat is a figure deserving of our pity; a rich person who kills to protect their position is a monster.
EDIT: Originally I said I would have more to say; as might have been expected Lord Peter also had more to say on motive which I came across on page 164. You can find it in the following blogpost which I imaginatively titled More On Motive.
 Currently on page 152 of 214. Recommended on what I've read so far.