Kiromaru at least appears to have some semblance of honor and pride in his servitude. But more importantly, he despises Squealer. He is more likely to abandon them if he sees the opportunity to kill Squealer than he his to betray them.

It doesn't really matter who gets their hands on the PsychoBuster, it couldn't be used to exclusively kill humans. It's Anthrax, biological warfare grade. It works against everything that isn't routinely vaccinated. There will be an inherent danger in using and handling it. I also doubt anyone other than the humans would be able to even produce a vaccine. So if Squealer took it and started using it all over the place, there would be nothing to stop it. Sooner or later, much of their livestock would die, and many of their people. Once the spores are released, there would be no stopping the slow spread.

At least until the queerrats end up with their equivalent of Louis Pasteur. This information wasn't input to mobile libraries if I understood Saki's mother's letter correctly. It was very controlled information.