Note: I enjoy Catherine Aird's work very much. I have just been thinking about trying to see how many weak points I can find in various mysteries, and happened to start with her.
In THE COMPLETE STEEL, the story is set almost entirely at the home of the Earl of Ornum. The house is a Stately Home (hence the alternate title, The Stately Home Murder), and is therefore shown to the public - that's one reason that the Earl can manage to maintain it.
On a nice summer day, a woman brings her two children with her because she wants to see the place, and her husband wouldn't have forgiven her for leaving the kids home. Her son gets away from her, and is later found messing around in the armoury, where he finds a body concealed in one of the suits of armour.
This, the first murder victim, turns out to be the Earl's librarian and archivist. Nobody had noticed yet that he was missing, because his sister was away, and everyone expected him to be at the big cricket match that weekend.
Various red herrings are tossed in front of the reader, of course. I'll ignore those and try to get to the point.
The reason the librarian was murdered is that a forgery had just been substituted for the most valuable item in the house, and he was the only person on staff who noticed the substitution. He was killed before he could raise the alarm - he'd telephoned the vicar to get a second opinion, and was killed before the vicar arrived.
Now for the plot's weak point.
Why was he killed?
The employee who turns out to have stolen the item had done something similar at his previous employer's, and the only death that came out of *that* was that his employer had a stroke upon finding out about the thefts. That is, he wasn't in the habit of killing to cover things up; he didn't need to.
If the employee had sat tight, and had gotten the stolen item out of the way, he'd have been a suspect but there wouldn't have been proof. The pattern - employ this person, and you start having forgeries substituted for valuable items - would have occurred to the cops (and did, in this case), but that in itself isn't enough to get him convicted without finding the item itself.
The first murder in this case didn't even buy him enough time to get the stolen object off the premises, as it turned out.
If the employee had sat tight, and had *not* gotten the stolen item out of the way, then whether he got into trouble would have depended on whether there were suspicious fingerprints or something on the item. Dusting for prints in the area where the item was normally kept wouldn't have been enough to prove anything if he kept his head - he was supposed to have been getting a new special lightbulb for that area anyway, and could have justified having fingerprints around the area.
If he'd been sacked on suspicion, in real life it wouldn't have wound up on his work record (at least, I think his lordship's solicitor would have advised against it).
In other words, based on the thief's previous record, I don't see why he would have committed premeditated murder to cover up *this* theft. The stakes weren't *that* high, and it isn't as though he just slugged the victim without thinking.
Since this is a motive weakness rather than a method or means weakness, however, it could be handwaved if need be.