It gets more people to die on ALL sides which is why it's considered a shitty thing to do, even if it's technically a "valid" tactic in some anarchic philosophies. The use of this ruse will cause people to stop accepting surrender which is BAD because it means that the people who die will be the average Joe and average Ivan in different countries who really don't want to kill anyone and just want to go back to their family and their life. These goodhearted people just want to go home and would gladly surrender. Imagine this thought experiment. I'll use your pen name dragonwriter as the name of a hypothetical super-human bandit / assassin / mercenary / gun for hire who sometimes does jobs like assassinations and Mission Impossible style burglaries of royal treasure. Imagine the DragonWriter bandit has no qualms about killing and so anyone who stands in his way gets shot with the intent to kill, AND now there are some rumors that a group of cops had DragonWriter surrounded and so he surrendered to them but then he pulled out a hidden sword and spun in a circle chopping them all in half. Now, any time DragonWriter tries to surrender in the future, his surrender won't be accepted, but what's even worse is: his action that bought him maybe a 5 second advantage (vs a million different other ways he could have won or fled the fight), and this tiny temporary advantage by the ruse has imposed a cost in exchange:
This one infamous sword fight has now poisoned the well with respect to the global norm. Next time there's some other totally unaffiliated hat-wearing super-powered bandit who's stealing the Ark of the Covenant or the royal diamond or something, if THAT bandit tries to surrender, there's a high chance that person might be killed, because their surrender is distrusted since DragonWriter changed the norm and now every fight between supers and cops is now on "shoot to kill, no prisoners, take no quarter" stakes. Imagine how many times a famous super like Batman would have been killed if the authorities never accept a surrender because the surrender norm is ruined. It can be repaired but that might take a decade or more to repair the trust that was broken.
With strong norms about surrender, red cross medics being protected, etc., we can have things like the miracle of the WW1 Christmas Truce which help the war to be a little bit little less of pure hell, for all the pure-hearted or indifferent people dragged into being a soldier, perhaps even drafted or press-ganged, to be the disposable cannon fodder for a bloody and disease and gas drenched war for no good reason more than rich politicians' egos.
https://en.wikipedia.org/wiki/Christmas_truce
These people just want to go back home to their wife or their mum and would gladly gladly surrender if given the chance, but that's not possible if someone ruins the norm! It's like, for example even if someone is a super-villain for example, they should really not take human lives of innocent people, or destroy irreplaceable cultural heritage sites, or damage/burn an ecological area like a rain forest, because those, once lost, are forever gone, and leave our world impoverished.