If all that happens in the world is to the ultimate Good, then why fight evil? Resist it, endure it - yes - but why fight? Isn't that like fighting God's will?
The short answer is that Christians should fight evil, because the evil in this world is from Satan, not God.
And, although Satan (like everything else) is from God, in this world we are (obviously!) meant to fight Satan and his servants (angelic and human) - this is spiritual warfare.
Equally obviously, in this world although we may win battles we will lose this war, because we are weaker. It is a long defeat, ending in the end of this world.
But we are meant to fight the long defeat, and that is service to God; since the only alternative is to serve Satan as accomplice, slave or dupe.