It will not occur to Keltham at any point that there is anything at all odd about continuing to follow his friendly-trading deontology, never using the trades of a friendly trading partner in a way that they'd hate, while he is plotting to destroy all the Evil gods / all the ancient gods / all the gods / possibly the multiverse.
Negative utilitarians don't particularly violate deontology? Dath ilan's negative utilitarians negotiated honestly, held to their bargains, and quit the field in an orderly fashion.
Negative utilitarians are famously scrupulous about that sort of thing. High scrupulosity appears to be part of the neuroatypical package. That they're even more honorable than average is part of what makes them such tragic literary figures.
If a dath ilani novel depicted a world-destroying supervillain as violating deontology in the course of doing that, everybody reading it would have been outraged at this enormously unjust straw caricature of negative utilitarians. The book would have been promptly condemned to the furthest dark corners of the Ill-Advised Consumer Goods store where they keep things as awful as biased political depictions. This has never actually happened in dath ilan.