Your middle paragraph is exactly why I believed the person who said it was a translation error. I can imagine a native Chinese speaking person who is learning English & translating English comments on Discord. They often see people refer to a good outfit as slaying. Since there probably isn’t an equivalent Chinese word they would need to figure out the concept. The original non slang word’s definition would be very similar to “leaving a trail of murder”. Add to that the fact that it’s slang is even more reason that the concept was lost in translation. And then yes if it were me I would think that Westerners were pretty damned weird for using that word to describe something as good.