ANSWER: You are confused as to what the Watchtower actually teaches, which is understandable. Allow me to explain.
Prior to the July, 2013, Watchtower, Jehovah’s Witnesses believed that the slave had been appointed over all the master’s belongings and an evil slave was put out of God’s spiritual house back in 1918. Now, though, the Society teaches that the master comes in the future and the appointment of the slave over all his master’s belongings has to do with the chosen ones receiving their heavenly reward. This is the correct view.
However, it should be noted that there are two appointments. The first appointment is when the master assigns his slaves to feed the domestics their food at the proper time. Obviously, that appointment comes first. The second appointment is a reward for having faithfully carried out their original assignment to feed the household.
Hence, when Jesus posed the question: “Who really is the faithful and discreet slave whom his master appointed over his domestics…?” – The answer is, it will not be apparent whether the slave has been faithful or unfaithful until the master unexpectedly arrives like a thief in the night. That is when one a faithful slave an an evil slave will be distinguished.