Jesus asked who “really”… because He knew well that there would be a claimant to the ‘title’.
Yes, they claim to be ‘the faithful slave’ but “who really” is.
Then Christ says “if THAT evil slave” thus identifying the claimant as truly an imposter and a charlatan.
Why, because the evil slave was beating his fellow slaves. Just as an evil house master would beat the slaves to extract more labour and to harshly impose his will upon them, the “evil slave” demands DO MORE DO MORE DO MORE and it’s never enough as they will lose out on life if they do not do more and more. And the evil slave further beats saying ‘you must refuse blood and die’.
Also, the evil slave says “the Master is delaying”, and how can that be said unless first the evil slave prophecied the time of the Master’s arrival and which time has passed.
There is no doubt, the watchtower’s leading men, have found their place in prophecy indeed.