Kryten has to save Mr Lister. The Low version of Mr Rimmer has taken him, and is probably doing who knows what to him at that very moment.
Kryten follows the sound of Lister's screams into the Drive Room. Oh, he's arrived just in time! Low Rimmer has him strapped down to the table, and is whipping him with gusto.
One punch from a solid mechanoid fist, and Low Rimmer is down for the count. That puts an end to that kind of behaviour. Kryten sets about untying Lister, and carries him to the medibay.
But it couldn't really have been disappointment he saw on Mr Lister's face, could it? Surely not. No, his facial recognition unit must be malfunctioning. It must have been gratitude.