"(a) display vigorous, active hostility to human life, civilization, and/or spacetime, and (b) are capable of causing significant destruction in the event of a containment breach."
"A hand grenade with the pin removed may be considered Keter: a single mistake will cause casualties."
"An example of a Safe object would be a gun, a nuclear weapon, or polonium."
My gut sense is that a Keter-class object should be 1) hard to contain and 2) capable- not just likely to, but capable of- causing an end-of-the-world scenario of some kind. The top ten current Keter-class objects mostly fall into this category. One presumes that SCP-682 would deliberately release other self-replicating Keter-class objects out of sheer spite, despite its general inability to destroy humanity under its own power. SCP-902 I'm not sure about- is "there is a cursed box that will afflict you with paranoia and other evils if it is opened" enough to trigger its effects? If so, you just need to mention the legend of Pandora's Box to someone to destroy their mind.
If Keter objects don't need to be existential threats to humanity, are measles or smallpox Keter? They are very hard to contain and will kill a lot of people without specific, complex preparations taken beforehand.