Kim and his producers could leave the film in its uncut state, but that would mean it can only be shown in "adult theaters". Were this the US, that may not be a slap of death, but South Korea actually doesn't have any "adult theaters", so Kim has no option but to recut the film before its release on August 11th.
So what will be lost exactly?
From Variety: "In "Devil," it objected to scenes showing a dead body being cut up and human parts being eaten by protags and a dog. "We will modify the duration of some of the scenes to respect KMRB's decision without losing the essence of the film," said Kim Hyun-woo, producer and CEO of production house Peppermint."
So it sounds like the scenes won't end up entirely on the cutting room floor, they'll just be shorter in length. Hopefully that means that the uncut film will be released in other markets (Optimum has already picked it up for the UK), but Jee-woon Kim's cuts have been hard to get a hold of in the past (good luck finding the 45-minute longer cut of A Bittersweet Life for less than $80), so if I Saw the Devil does get released with all the violent bits intact, I'd recommend jumping on it right away.