killall -r -9 . sends the 9 signal to all processes matching the . regular expression (i.e. it kills all processes). As it kills itself, the question is, will it kill itself last, therefore doing what it is documented to do, or maybe it will kill himself before finishing the work, leaving processes alive.
Following @David’s answer, it means that if you run killall twice it has less chances of working than if you run it once – because each killall can kill the other before it finishes working.
Is this a new paradox, akin to the halting problem?
The man page says that
killallwill never kill itself.