Zed did nothing wrong.
I have recently taken a liking to Zed as a champ I play, and it has recently struck me that Zed was basically a normal ninja, in addition to the fact of being a more realistic ninja in terms of his style. Now, no discredit to the anime loving creators at riot, but contrary to popular belief ninjas are not tanks, nor are they very honorable, and they sure as hell aren't aoe killing machines (It's a video game I know, but if it's a magic ninja at least make it a ninja please?) in reality. Zed is a single target assassin in the sense that he marks ONE DUDE and kills them, as close as you can get in a MOBA. Anyway, our story starts with Jhin's killings. Zed wanted him dead, like a reasonable human. Proof was there, Jhin had every right to a death sentence. Next we have their training. Zed and Shen were even, zed found a shadow box and it gave him an edge, LIKE ANY REAL NINJA WOULD WANT TO HAVE, DUH. But Shen's dad took it as cheating, and evil when THE SHADOW POWERS HAD NOTHING SINISTER ABOUT THEM YET. So he banishes Zed. Now Zed, being banished by some nerd who doesn't know how to be a ninja, gets mad like any person would, and wants the box so he can train himself in the shadow tactics. Where he ultimately kills shen's dad. And now he trains people in a "learn or die" way which, I'll admit is a bit cutthroat but a real ninja would want to learn how to be uh.. idk.. ONE WITH THE SHADOWS ANYWAY? So what did we learn here? Zed wanted to get better, so he learned better moves that had no clear drawbacks, and Shen's dad is an idiot who is too focused on fairness. When he's a ninja. People who literally fight "unfairly" (Stealthily) to win. Ninja does not beat samurai in an honorable 1v1. Ninja beats samurai in a cheap shot.
TL:DR Zed is closer to an actual ninja, but is a bad guy for it. Shen and all them filthy anime fanfic characters are all designed based on a far fetched fantasy outlook that ninjas are honor slaves, and Shen's dad was the whole problem.
If I'm wrong here, please give me an extensive explanation on how shadow powers work. Because rn they just look like a technique that idiots who call themselves ninjas think is bad.
Not trying to make anyone mad here, any seemingly insulting phrases are just banter.