People have been known to do that sort of thing. You're thinking that people don't keep up old jealousies for twenty years or so. Perhaps not. Not just primitive, brute jealousy. That means a word and a blow. But the thing that rankles is hurt vanity. That sticks. Humiliation. We've all got a sore spot we don't like to have touched...This is bolstered by an authorial footnote:
Lord Peter was not without authority for his opinion: 'With respect to the alleged motive, it is of great importance to see whether there was a motive for committing such a crime, or whether there was not, or whether there is an improbability of its having been committed so strong as not to be over-powered by positive evidence. But if there be any motive which can be assigned, I am bound to tell you that the inadequacy of the motive is of little importance. We know, from the experience of criminal courts, that atrocious crimes of this sort have been committed from very slight motives; not merely from malice and revenge, but to gain a small pecuniary advantage, and to drive off for a time pressing difficulties.' - L. C. J. Campbell, summing up in Reg. v. Palmer, Shorthand Report, p. 308. C.C.C., May 1856, Sess Pa. 5 (Italics mine. D.L.S.)(This is from a famous poisoning case, apparently. I have tracked down some other citations; copy and paste into the search engine of your choice for more.)
So here Lord Peter and (for the purposes of fiction at least) Dorothy Sayers agree with Lord Campbell, that although we must have a motive, it does not have to be a compelling one. Our criminal does not need to be forced to logically commit the crime as the best of a set of poor choices, it merely has to be one that provokes that particular person. Insults or difficulties that another character might ignore or tackle differently lead this character to murder (or steal, kidnap, burn down, blackmail etc.)
My own conclusion (not wishing to put words into the mouths or writings of Sayers, Lord Peter or anyone else) is that motive stems from character at least as much as circumstance. Some people might attack when threatened or provoked to their face; others will harbour hatred of what others would see as minor slights. Match crime to character and character to motive. Of course this all depends on where you start; has one created a cool person who you want to commit a crime? Or do you have an especially compelling murder and need to figure out why someone would commit it? Can one begin with motive and derive crime and character fromthis seed?
Personally I find that actually writing warps all plans; as character comes into focus it transforms the motives and crime; focusing on getting the mechanics of the crime right changes character and thus motive. Edit, re-edit, draft and re-draft until after enough iterations they come together (or not in some cases).