So for executions, it is improperly prepared then.
Exactly.
Maybe there is a traditional 'poisonous' preparation?
Probably it's a highly-ritualized version of doing it wrong-- ritually skipping a certain specific step.
Now, making tea is, in general, one of the simplest ways to prepare much of anything. Scalding water is either strained through fresh or dried leaves or has same dumped into it. It's hard to get wrong.
... Which means that "doing it wrong" must be something interesting. Thoughts:
* It can't just be fresh versus dried; that's too hard to screw up.
* If the poison works like the tannin in black tea, allowing the leaves to remain in contact with the water for very long turns it deadly. To make it properly, the scalding water is poured through a cloth (NOT METAL-- the holes have to be too small for bits of leaf to pass through) strainer, which probably contains large chunks of shredded, dried leaf. Properly prepared, the tea is pale; improperly prepared, it might be dark, and may have bits of kresh leaf floating in it.
* If the above is the way it works, I'd expect typical, minor mispreparations to usually be pale but to have leaf fragments-- the one warning you get. Toxicity would be closer to human tolerance, if not necessarily much more survivable (does it really matter whether the muscles you breathe with are just paralyzed or really, really paralyzed?). I'd expect purposeful ones to be darker (water added directly to a teapot of dried leaves in the manner of the black tea that comes to your table when you're having a dim sum lunch), and also more toxic.
* If the poison is flavorless, even a ritual cup of deadly hak'len tea need not be a bitter draught-- in fact, it might even be infamously sweet.
* Alternatively, part of preparation might be dissolving out the poison. This likely involves soaking the dried leaves in something prior to making the tea-- brine, perhaps, or vinegar. This, however, would flavor the tea and would likely result in commercially available "processed" kresh leaves that have had the toxin extracted prior to packaging. It would also possibly change the flavor, making properly prepared hak'len tea a little sour or salty, or something of the sort. It would also probably dull the flavor somewhat. Improper preparation, in this case, would probably be about the same color but taste a little different.
* As another alternative, the poison might reside in only certain parts of the leaf, which would need to be carefully removed. I'd guess poison might be concentrated in the central vein, stem, whatever the hell a botanist would call that bit of the leaf that forms ye olde main road / structural support. This has to be carefully removed, like a blowfish liver. Kresh leaves in this event are probably used fresh, not dried (otherwise the leaf could be carefully cut and inspected prior to drying), and trimming out the central stem of each leaf is part of the preparation process; no fragment must remain. Intentionally toxic tea would be made with the whole leaf.
Upon reflection, I think I like that last one best. Thoughts?