Basically, the rule wouldn't be that "Charas that harm others are out of bounds". The rule would be "Charas that harm others are X-Charas".
That could have been the case here as well. It would still prevent them from actually becoming charas, and I need to point out that -- Amu's speculation aside -- all she knows is that she's never seen a chara like that, not that they don't exist. People with violence-
adjacent charas, such as Kairi or Nadeshiko -- or Amu herself to a degree, via Su -- tend to be paragons of virtue. Yes, I'm including Kairi in that; he struggled, then actually went against the wishes of his older sister, whom he loves a great deal. That's not easy at all.
(He's now Utau's uncle. Oh my.)
But yeah, my post was about this quest. Not Shugo Chara canon, and your point is valid; it'll be interesting to see what Peach-Pit actually go with, assuming we get an explanation of any sort at all.
To give you some of my reasoning...
I don't believe in magic. Objects can behave in complicated ways, and rulesets can be complex, but they can't be
fundamentally complex; the complexity is always emergent. This is true even for something as seemingly fundamental as the proton, which has been described as
'the most complicated thing imaginable' -- I disagree on that, by the way, human brains and biology is worse.
Then, what's the explanation for the magic in this story? It's not faked; Amu absolutely has telekinesis, Miki exists. There just has to be an explanation.
I've already hinted that it was deliberately created, and this is -- true, although the jury's out on by
who. But in addition to being created, the behaviour of the Chara system is itself complicated enough to demand intelligence. That's not a major problem, as there's a gigantic sea of indistinctly attributed intelligence floating around in the form of the collective unconscious, but it has implications for how it works -- in this case that most of the rules for the 'Chara system' (and wow, they'd like it to be controllable enough to deserve that name) are best described in terms of English, not fundamental physics.
That being the case, and considering how it was made, many of the rules have to do with safety. Safety for who? Well, that's an interesting question, but if you ask about the purpose of the system you'll get a different answer depending on who you ask. Lulu would describe it as "Letting kids achieve their dreams, and
keeping them alive so they can do so."
She'd then go on to drive them insane while blindly overcharging their chara, so they can achieve said dream immediately instead of-
<Lulu> I wouldn't do that!
<Amu> You
did do that.
<Lulu> I'm not ten years old anymore, Amu.
Hotsuin, who spent nearly an entire evening interrogating her last week, has a far more cynical viewpoint. And wants to know when precisely she was going to tell him she's soul-bonded to a dubiously benign fairy-like creature.