The letter that talks about how the "cross" Saki found is a tool of exorcism. (It's not in Japanese so Saki has the library critter scan and translate it.) It calls PK the devil's gift that will lead humanity to its fall. Absolute power corrupts absolutely, and a power too great for humans will eventually destroy its user and his surroundings. Thus, it's the power of evil, and the people it possesses are demons and witches. They should not be allowed to continue their evil ways, and lo the psycho buster is just the way to stop them, it's truly God's blessing. In the old times pagan terrorists used to send this "holy powder" to their targets, but for exorcists on a holy mission it's more fitting to use it like a Saint Benedict's medal, hence its shape, a cross. Hallelujah.
(It also explains how to use it: you slam the container at the ground at the target's feet, and the inert gas inside will disperse the powder.)
At this point Saki realizes that there's powder inside the "cross" (she didn't notice it earlier) and what she thought were horns actually form the biohazard mark. You can see vague traces of this in the anime design. And she reflects on how sick and twisted this whole thing is, and yet it's her only hope.
Obviously, this is not crucial to the plot, but it's pretty chilling, it explains what the psycho buster is doing there in that safe and why it looks like the way it does, and adds to the horror Saki is going through. In the letter the power that Saki's people celebrate as "the power of the gods" is called "the power of evil" and "the gift from the devil"
(a cursed power - this is the pun hidden in "juryoku," there was no need for all that "Cantus" crap) and the idea will come back to haunt Saki later as she ponders whether it has done more bad for humanity than good.