Paul plainly tells them that the Judaizers have perverted the gospel or turned it into something else. These teachers have not taught the gospel that Paul taught, but a different gospel, and one which he rejects.
We may be clear on the issue raised by the Judaizers, but Satan does not cease to attack the church and re-use old strategies in slightly different forms. Do we have the discernment to see them when they come in a new guise and to warn others?
Just as he was not prepared to say anything good about their doctrine, so he was not prepared to say anything good about the Judaizers themselves. They were troublers who he regarded as enemies of the churches. But didn’t Christ command his disciples to love their enemies? Yes, but when they were attacking the sheep and lambs of Christ, and there was a danger of real harm, it was not the time to be showing friendship. The Galatians needed to see these enemies as a serious threat; if Paul had been treated too gently, he would have made the danger seem small. When believers are confused about who their friends are and who their enemies are, then they need clarity on the situation. Once they are clear on this it may be time to help even the false teachers, but even so our approach to them is going to be completely different. We can never act so as to suggest that we recognize any truth in what they say. Love will cause us to speak very plainly to them, and to leave him in no doubt that they oppose the truth of God.