Quite simply, the "Evil servant" is the Russellites who weren't buying into Rutherford's new program...the "faithful servant" in contrast were the "loyalists". Down through the 1940s the WT blasted this "evil servant" at every turn and it was still referring to Bible Student groups still loyal to Russell. Now the fact that a definite "evil servant" was identified in 1918 is glossed over in favor of a generic "evil servant".
The Watchtower, Nov 1, 1944, pp. 332-333
Fulfilled prophecies make it certain that Jehovah’s Theocratic Servant Christ Jesus came to the temple in 1918. Then he entered into judgment with the consecrated servants of Jehovah. He did so in order to determine who of them should be retained in God’s service and be constituted and organized as His "faithful and wise servant" class to do the evangelistic work thenceforth. The servant’s responsibility and duty is to dispense the spiritual food as the great Theocrat gives it in due season by the unfolding of his written Word, the Bible. The history of the past seventy years discloses that the heavenly Father and Food-Provider has by-passed the anti-Theocratic religious systems and their hierarchies and clergy...
...During the period from 1878 to 1918 Jehovah’s devoted servants were endeavoring to act unitedly in doing the witness work pictured by that of Elijah the prophet and hence referred to as "the Elijah work". Like Elijah’s work, it was against religion or demonism and for the vindication of God’s name. This work, conducted under the leadership of Christ Jesus, came to an end in 1918, when he came to the temple as Jehovah’s Messenger and Judge. (See Malachi 3: 1.) The judgment tests which he then and there applied separated out from Jehovah’s professed servants an "evil servant" class, which followed after man-worship, selfish ambitions, and self-righteousness, and which aimed after the control of the governing body.
A remnant manifested purity of heart and devotion to the Lord God and to his Theocratic organization and the interests of his kingdom. This faithful remnant was approved under the judgment trial, and Christ Jesus the King gathered them unto unity with himself at the temple. He adjudged these faithful anointed servants of Jehovah God to be the "faithful and wise servant" class under himself as Head. To them he committed the interests of the Kingdom. That is, he laid upon them the duty and privilege to carry on the evangelistic work as Jehovah’s witnesses in ’preaching the evangel of the kingdom in all the habitable earth for a witness to all the nations’. This they must do, down to the battle of Armageddon, when the Kingdom will destroy all the enemies of The Theocracy and then the millennial reign of Christ Jesus will begin. This "faithful and wise servant" class is the real "Society" of Jehovah’s anointed remnant...
