There was still a chance that Frederick would also be executed...Katte’s death, however, announced a slight lightening of the regime in Küstrin, and a minor rehabilitation for Frederick. Frederick William had assuaged his blood-lust. That, and the problems which might beset the succession, convinced him to spare Frederick, rather than the plea for mitigation that he had received from the emperor in Vienna on 1 November.
Which is consistent with selenak's case that if Keith, then (probably) not Katte.
Re: Peter Keith
There was still a chance that Frederick would also be executed...Katte’s death, however, announced a slight lightening of the regime in Küstrin, and a minor rehabilitation for Frederick. Frederick William had assuaged his blood-lust. That, and the problems which might beset the succession, convinced him to spare Frederick, rather than the plea for mitigation that he had received from the emperor in Vienna on 1 November.
Which is consistent with