This one looks interesting, so I'll add my 2 cents to the three you mentioned.
Varus: He's not evil, but corrupt in mind and body. What was Varus, the temple guardian, before, died with his village. Now, he's vengeance with a bow, out of kill all of Noxian invaders who ever set food on Ionian soil.
Syndra: She's not evil, never was, even after killing her mentor. In fact, it's for this reason I began my fanfiction of her and Xerath. As a little girl, she wanted to become stronger for some reason, I think to be a powerful sorceress to make her parents proud of her. She was excited to train with a master mage, believing it would help her achiever her goal in life. Instead, she was betrayed, lied to by her parents and village elders. Her mentor was only the target of her rage because he did not tell her. She trusted him, lived in the temple turned school with him. After she killed him, she did not go on a rampage, but fled, taking the building with her. Now, she can trust no one, but those who are stronger than her and able to defeat her.
She's not evil, but suspicious and untrusting of everyone.
Zed: Just like Varus, he's corrupted. That box is evil. EVIL! I don't know how old he was when he first made contact with it, but it's clear his master was a fool to not seal it or put a "Evil spirit! Stay away!" sign on it. As far as killing his master goes, it could be that what was inside the box fully took control of Zed and is using him.