Oh please if he'd put that sign there other kids would've opened it before Zed. If someone tells you not to touch it you will touch it. His master should've hidden it better.

In either case Evil is a objective thing that is pretty hard to pin point, the closest you'd get to traditional kick puppies for fun evil would probably be Zed, but even that can be argued.

I think his Shen's dad should've given the secret to someone trustworthy to master the technique and teach it to others, but in the name of good.

Think about it. The first lessons would be about learning what a dangerous technique it is and the need to preserve it within themselves, otherwise it would spread and cause chaos.

And if there was one psycopath to get past the watchful eyes of the masters and learn the technique anyway, there'd be a higher chance of stopping him since there's an entire school dedicated to not only learning it, but keeping it under control.

Or not, seeing as outnumbered ninjas always win. But it still makes more sense than hiding it.

The box, isn't evil, it's just that the Ionian Elders considers its contents "forbidden". Zed took a peak inside and saw those awesome skills, learned them and defeated his rival, then people told him "No Zed wtf that's forbidden" and sent him away.
Then he got angry and made and army, came back, got his revenge and whatever...
Zed is a Sith and Shen is a Jedi, basically. Though Zed is mostly portrayed as an evil shadow ninja supremacist and Shen as a... Neutral ninja of balance (though to this day, the lack of negative acts performed by the Kinkou makes it hard to call them entirely neutral or anything).

Same thing with Syndra really, who used to be a pretty normal person, albeit very powerful and liking it, but like they hid the forbidden ninja skills, they tried to slow her down. I cant remember exactly, but their intent was to make her evolve more slowly to avoid blowing things up hard, not to actually "seal" part of her power forever. Syndra's reaction is justified currently, as they did slow her down for years and we currently don't know of any negative effect of her rapid power growth. They could have told her but they preferred to keep silent, their loss.
So yeah, again, free will versus restrictions, which is more of a chaotic versus loyal than evil versus good.

Varus' case is... Well, he was definitely good and loyal to Ionia, following his duty until the end, but then, because of this decision, everyone he cared for died. It's unknown if it would have made a difference if he went instead of staying, but he felt powerless, and it's understandable. Then he decided to be stupid and got himself corruption all over. "I have nothing to lose so I'll take revenge anyway".
Now he's just a darkness goop corrupted symbiote bent on revenge. Kind of like Kratos in God of War, except technically more likely to become completely crazy in the end, then he'll be an evil darkness puppet. For now he's still sentient.

Now on the different case of Swain, he's evil because he's a dirty politician in a land of ruthless warriors. He makes shady deals, assassinates leaders and is more of a mage than an actual fighter. This is the kind of people Riven doesn't want in Noxus really : leaders who likely won't show up, heading their armies and fighting their all like everyone else. I don't remember his true intents though, is it ruling over Noxus and making profit/conquering others (somehow) or being a leader his nation can look up to? More of the earlier than the later really (afaik).

You know, writing this makes me think... Those Ionia champs mostly lack character development. Swain had plenty of occasions to be shady and opportunist in the JoJ, whereas Varus, Syndra and Zed are all relatively new and limited to their origin story.
I really hope we'll get more one way or another.

Varus took a drastic measure to ensure that the Noxians will pay at the cost of obviously his life/sanity.
Syndra was a moody teen who had way more power than she actually needed/could use that threw a tantrum when they took her toy away.
Zed was looking for a way to win for once. He obviously opened Pandora's ninja technique box thus it made him into what he is now.
So I guess there are no real Ionians who are evil.