Zed "was" a nice guy
Zed is a nice guy and honestly I would had reacted the same way
"young Zed marched forward to kill the cowering man, but Kusho held him back. Despite the horrors of Jhin's actions, the legendary master decided the killer should be taken alive and left at Tuula Prison. Shen disagreed, but accepted the emotionless logic of his father's judgment. Zed, disturbed and haunted by the murder scenes he had witnessed, was unable to understand or accept this mercy, and it is said a resentment began to bloom in his heart"
Now i Know what started it