So, there's TWO things that could have gotten Kim killed...
- Trying to bind a sufficiently-dangerous creature in the circle (a circle she's neither competent-enough to create nor strong-enough to empower) and failing.
- The Wardens finding out she's messing with Outsider/Demon/Elderthing caliber circles, and snicker-snak'ing her.
Harry DID warn her off of #1... not explicitly saying WHAT she'd found, but that it was dangerous, and beyond her; arguably, he could/should have said "something strong enough to need this circle is too strong for you to contain it, and it will just break the circle and kill you," but he didn't say that.
It seems he left #2 un-touched, which I put down to EIW (but as noted upthread,
could also be how horrible Harry found it, to deal with the WC/Wardens/DamoDoom schtick; a blind spot by virtue of his own trauma, crossed with a severe version of "keep them ignorant for their own good" that Harry does for the first 10ish novels).
Looking upthread, fwiw, I think some folks are talking at cross purposes regarding those two points.