Having warned the church at Ephesus of their lampstand being removed if they don't repent, Jesus then says,
Yet this you have: you hate the works of the Nicolaitans, which I also hate. (Revelation 2:6)
They had this going for them. They hated the works of the Nicolaitans. We're not sure what these works were exactly, but perhaps it was some sort of antinomianism, meaning they taught people that they could live lascivious lives. Verses 14-15 seem to connect the Nicolatian teaching to the teaching of Balaam, "who taught Balak to put a stumbling block before the sons of Israel, so that they might eat food sacrificed to idols and practice sexual immorality." Whether the Nicolaitan teaching was also encouraging idolatry and immorality is not clear, but what we can see is that these two teachings could comfortably coexist together.
In any case, the Nicolatians produced works that the Ephesians hated. The Greek word here refers to having a strong dislike or opposition toward something. Here's one example where it shows that hatred is 100% an appropriate attitude for the Christian--provided it's directed at the right thing. Jesus doesn't say that the Ephesians hated the Nicolaitans. No, they hated the Nicolaitans' works. And, of course, they should.
Whatever the Nicolaitans were doing and teaching, it was contrary to what Christ taught the church to do and believe, and therefore it was dishonoring to Christ and destructive to people. Why would we not hate those things?
This is why Christians must not be "allies" or "affirming" of people who sin against God. Even if the person is an unbeliever, he is still obligated to obey the law of God, which is honoring to God and for the flourishing of humanity. Rather than affirm sin, we ought to hate it. Loving neighbor includes hating sin.
By the way, Jesus hates it too. Notice the word "which I also hate" (v. 6). Jesus hated the works of the Nicolaitan, just as He hates all falsehood and disobedience. And following Him, we ought to hate them also.
Understand also that we are nowhere in Scripture commanded to hate the sinners. In one place (Psalm 139:21-22), the psalmist does express his hatred for those who hate God, but still, that's not permission for us to do so. On the contrary, rather than seeing commands or permission for us to hate sinners, we see that Jesus teaches that we are to love our enemies and pray for even those who persecute us.
So, we're not to hate sinners. But we are certainly to hate sin and falsehood. A failure to hate sin and falsehood is rooted in a deficiency of love for holiness and truth, which ought to be rooted in love for God, who is holy, holy, holy, and true.
May God help us to rightly hate sin and falsehood.