While one could argue that Darius is a villain, a blood thirsty king who wants an ever lasting war .... he treats his people well. Notice how in Garen's introduction, the first thing he does is kill his own soldier without mercy. When the time for both sides to collide, who's the one leading his army?
Who's the one waaaaaaaaaaaaaay in the back, relying on his frontline to hold off a god-tier rival? Garen is. For a man who claims himself to be all powerful and a God, he sure likes throwing his soldiers under the bus. If you want to win a war, you can't kill off your own men and also expect to have more in the end compared to the opposing side. Darius treats his people well, and takes out the problem so they have the advantage in the battlefield.
_"SO why side with Darius though?"_
Unlike Garen who'll watch you die to please his status, Darius will **join you in the battlefield and help you fight.** That's the man i'm willing to place my trust in, who's willing to risk his life for his cause, unlike Garen who's a selfish tyrant.