"Question for the experts: Wouldn't the bacteria, at the point of encountering survival pressure, become less lethal? For example, I know that lethal viruses evolve over time to co-exist non-lethally with host species, since killing your host is not a great long-term survival strategy."

From the intraspecific perspective, the bacteria that kill males will always outcompete those that don't within a given population, and they will increase even if it drives the host species to extinction. At the population level--if populations are discontiguous--then there could be a type of group selection if host populations harbouring the male-killer bacteria are driven to extinction and take the bacteria with them into oblivion, leaving only populations harbouring less badly behaved bacteria. What's more likely is that the hosts will step up to the plate and evolve a form of innate resistance. In a population that has almost no males, the males that do exist will be wildly successful and they will pass on their mothers' genes, so a female that can fight off a bacterium that kills sons is going to be laughing all the way to the...um...gene bank.....