Akuyaku Tensei Dakedo Doushite Kou Natta. The handsome Charlie torture prisoners for information, brainwash others, use prisoners as great war sacrifices, kill the enemies, and gets pissed when the shounen protagonist don't kill the enemies just because they are children.
Honestly, most Xianxia MCs rationally kill because the characters around the MC have almost no middle ground. Pretty much anybody involved is either good, or just an unrepentant douche.
The mc in Warlock of the Magus World is very calculating and doesn't need care for grudges. If it's useful, he'll kill. If it'll cause a problem, he'll forget about it
some i know thats kinda different are Hakomari katanagatari samurai x berserker manga Shield Hero maybe most korean novel ever some novels about people turning demon/apparation/evil spirit/alien monster
I thought about it and this series came to mind: http://www.novelupdates.com/series/my-disciple-died-yet-again/ It's basically a parody of normal xianxia tropes. Tho our MC is a little foul mouthed, she sticks to her modern principles and won't kill. My memory is bad, but I don't actually remember if she's killed or not in the series so far...? She hasn't killed any demonic beasts yet either, I don't think. Rather, I don't think she's really gotten a chance to lol. Ehhh... but, this should (probably) fit the bill. Even if it doesn't, it's a fun read. Give it a go if you can.
Yea the real problem is how the authors can't seem to be able to make any sort of antagonist that isn't some evil cultist/demon or a irredeemable douche that would develop a vendetta against anyone for the slightest perceived insult, then to go along with that is that even if the MC does kill the douche, douche's family or w/e group, sect, power etc. that they're related to will put the MC on the wanted list no matter who was in the wrong, then the MC has to kill them, then after he has to kill the women, children, babies and their pet rocks cause none of them are smart enough to leave the person that just annihilated half the country alone cause life preservation isn't a thing in these CN novels. Anyways give Rakuin no Monshou a shot.
The "real" batman is the one that lives in my heart(because he is imaginary and he can be whatever I want him to be), anyways there are various versions of batman, to know which one is the real one would be hard(or just look at the earliest release date, it would be easy), realy realy hard.
Chronicles of Primordial Wars Super rational, he will kill when he needs to, but he will spare a bad guy if he can make use of him another way.