You may think that legalists are strict about obeying the Word of God. That is certainly the caricature. Of course, that means that Christ is a legalist and the Pharisees are actually bastions of grace. This isn't the case, however, as the legalist is not one who desires that you obey the Word of God. Instead, he desires that the Word of God is undermined. He doesn't want you to obey the law of God, because he wants you to obey his own law.
Unregenerate men do not only want to be a god, they want to be your god. And when you don't approve of their rules, rebuke them for attempting such a game of hubris, and call them to submit themselves to the Word of God instead, you will surely feel their wrath.
And that is the true nature of the legalist. He may speak all about grace, but only because he wishes to undermine the holiness of Christ to which all are called to be conformed. And this is merely done so that he may insert his own behavioral ideals in their place, not because he needs necessarily obey his own ideals, or that he is even consistent with them, but rather, again, that you may be subject to his rules as his devoted followers.
This is why Paul warned the Ephesians to adhere so intently to what he had taught them, because savage wolves, not sparing the flock, will come because they seek to draw disciples after themselves. They replace Christ with themselves, and hence, they replace His law with their own. The true nature of a legalist, then, is not someone who wants to obey Christ, but rather one who wants you to obey antichrist, because his antichrist is himself.