Dokuhime

So that bastard was proving what she was by killing the maid. OK, that's really stupid on his part--"Hi, I'm a badass, I'll prove it by making you hate me and giving away a big piece of leverage on you. You'll for sure co-operate with me now, right? Well worth an unexplained death with obvious connections to you that might make someone suspicious." Really. Even if he really wanted to test it, why on someone she cared for--indeed, far as he knew perhaps the only person in the kingdom she cared for? How does that gain him an advantage? It's dim. But OK, maybe he's just that much of a bastard, whatever.
But also, OK, so the test was obviously going to work. Rather than just say "No, don't do it!" is there any obvious plot or character reason why she wouldn't just say "Yes, fine, you got me, that's what I am, no reason to kill the girl to prove it now"? Sure, possibly he would have killed the maid anyway, just to be a bastard or for security reasons or whatever. But is that a reason to just sit still for it?
Meanwhile, those princes know she's a poison princess, and that bastard apparently simply let her go after using her to murder an innocent in cold blood and earning her hatred. Why doesn't she just go to them and rat him out? He seems not to completely realize they've identified her, so he might think she'd keep quiet, but why should she?
The whole scene is full of gorgeous pathos but as soon as I thought for two seconds it made no damn sense.

It's a sad manga also the Poison Princess are supposed to be killed after doing their job aka Assassinating their target

some maid steals her used linen instead of burning it like she's told, and dies from the poison