Tikal's story allegedly began (more on that 'allegedly' part later) when she stumbled upon the alter of the chaos emeralds. There she met a group of creatures known as chao, and their protector, Chaos. She befriended Chaos and the chao as they lived peacefully in their obscure home.
A whole bunch of time later, Dr Robotnik learned about Chaos from a stone tablet or something, and thought: "hey, those chaos emeralds are pretty cool, but you know what would be even cooler? That Chaos fellow working for me. With all the power of the emeralds to give him the maximum potential of his raw might".
Sonic and co aren't too keen on this plan and try to stop him. The events of this were chronicled in 'Sonic Adventure': a launch title on the Sega Dreamcast (yes I'm talking about that thing again. Try to stop me dammit).
Tikal, as a ghost, tries to aid the gang in preventing Chaos from reaching full power by occasionally showing them visions of the past which teach them about all the previous bloodshed. But more often her advice comes in the form of her being this ball of light that hangs around maps in the game and gives you hints when you touch it.
Ingame, this was designed to be an immersive way for the player to know what to do next. But from a story perspective this concept takes a sinister turn when you realize that one of the characters who she helps begins their story working for Robotnik. Eventually he (it: the character in question is a robot) does switch sides, but up until that point Tikal was telling it to do things that would be extremely detrimental to the whole "peace" thing.
"Ah", the raging Sonic nerd on par with my own raging Sonic nerdom would say at this point, "but that's not necessarily true. Trapping Chaos was a last resort. He was her friend. She simply wants the game's events to take place so that he can eventually reform and be good again".
You're wrong, I would say, and am in fact saying right now, that doesn't make sense. When Chaos does reach full power Tikal suggests trapping him in the master emerald again, but Sonic rejects the idea, saying that it won't fix anything because Chaos will still harbor his loathing for basically all life on Earth... or Mobius... whatever.
And it can't be that Tikal is acting on some precognition of fate either, because why would she bother to disagree with Sonic at all? If she knows what he's going to say and knows he can succeed in quelling the beast's anger (spoilers by the way), what's the point in her saying anything?
Keep in mind that everything shown to Sonic and the others are visions of the past which Tikal constructs for them. They aren't seeing the past. They're seeing Tikal's construct of the past. It is very possible that she is distorting the truth or outright lying. Knuckles knows nothing about his ancestors or their history, so he can do nothing to vindicate her.
No, fellow boomers, Tikal's version of events are not what I suspect happened. Imagine, if you will, that Tikal did NOT survive that assault on the emerald alter. She was killed violently, and wants revenge. Chaos being angry and getting freed was just convenient to her interests.
Or worse, Robotnik's interests are exactly the same as Tikal's were. Pachacamac had nothing to do with Chaos destroying the world, or perhaps there never even WAS a Pachacamac: his supposed deeds were actually all Tikal's doing.
Tikal knew the legend of the chaos emeralds before she found them. She doesn't even try to lie about that. Maybe she wanted to control Chaos in the pursuit of world domination, just like Robotnik did, and just like Robotnik, Chaos turned on her. Only unlike Robotnik's, her mistake proved fatal, and so she had to modify her plans to the 'beyond the grave' variety.
"But Choas ended up relenting! Sonic steered him to the light!" No, no, he merely 'neutralized him'. Those were Tails' exact words: "Super Sonic must have neutralized him! So he's nice again!" If that sounds like Sonic decided to sit down with Chaos and have a chat about why genocide is a bad thing... that's not what happened, FYI.
What did happen was that Sonic, in his super form, just kind of repeatedly rammed into Chaos until he rethought the whole genocide thing all by himself. Tails' exposition makes it sound as if this was accomplished by Super Sonic's positive energy rubbing off on the monster and effectively brainwashing him into pacifism. And not in an inspirational way. As in radioactive energy literally broke Chaos' brain. He may have turned out nice, but that's not because he was nice to begin with.
Oh, did I mention that a bunch of the hero characters' efforts in some way backfire and benefit Robotnik in restoring Chaos in ways that never would have happened otherwise? Yeah. Remember who was helping them with that?
Tikal didn't know Super Sonic would be able to stop Chaos. She was thinking she'd have Sonic fight him, he'd die, and then there'd be no one to stop whatever nefarious thing she wanted to happen next. Yes friends, THIS is the face of EVIL: