to set oneself against someone or something: to oppose, resist, or stand firm against
The word is built from "against" plus "to stand," and that picture holds every time it is used: a person plants themselves in the path of another and refuses to give way. The single image faces two directions, which is why English needs more than one word for it.
Sometimes the person is the aggressor, taking the initiative to confront or thwart. Paul says he "opposed" Peter to his face; Elymas the sorcerer "opposed" the apostles to keep a governor from believing; Alexander the metalworker "strongly opposed" Paul's message. Here the natural English is "oppose" or "withstand a person."
Other times the person is on the defensive, holding their ground while something bears down on them. Believers are told to "resist" the devil so that he flees, and to "stand firm" in the evil day. Here the natural English is "resist" or "withstand." A near neighbor, ἀντιτάσσω, pictures the same posture as lining up for battle against an enemy, and the two words appear side by side when Paul warns that whoever resists governing authority is really opposing what God has put in place.
Because the same verb covers both the person who attacks and the person who holds the line, the moral weight depends entirely on what is being opposed. Standing against God's declared will is futile and culpable; standing against evil or against the devil is exactly what believers are called to do.