@Atmosk >slowly sliding towards becoming evil
She literally mindfucked her favorite person, stabbed a hand to make a point, chopped off an arm out of casual disgust and disregard for life, emulated the sociopathic tendencies of her former master who she tried to steal techniques from, played accomplice to the actual protagonist's multiple and completely ruthless acts of murder, and you're still going on about a slide?
She is and has from the beginning been fully evil, sitting very, very close to the moral event horizon with no signs of redemption. The MC too. None of what you consider bad are bad things in this universe. The rules of any given fiction depend on the author and this is clearly one in which nobody really thrives, ever, by being a good guy, so following your advice would be objectively a dumb thing to do.
It doesn't matter if an opponent represents a small threat because that's still a threat, so it should be removed, with the added reason that not killing her would mean Tao Hua comes across as weak in a place where ruthlessness holds merit. I've also made plenty clear why she'd personally gain satisfaction from killing her, so I still don't see why it's so hard for you to accept how much merit there is in just chopping her head off and removing a bad variable from the equation while attaining some vindication in the process.
Matter of fact, since she's an enemy, the burden falls on you to prove there's merit in sparing her from where I'm standing. Actual merit, instead of your personal ideas which are clearly not held by Tao Hua or otherwise relevant to this particular manhua in any way. Nobody here cares about the sanctity of life, some arbitrary sense of justice, or something as counterproductive to survival as fairness or honor. This is what her character has been established as and what we've come to like her for, and it's clearly what should keep happening.
At some point you gotta realize the difference between valid criticism and opinionatedness. Don't read NTR stories if you're angered by adultery, and don't read amoralistic stories if you can't enjoy a bitch being ice fucking cold.