What is wrong with killing Sasuke off screen???

I've never understood why some fans seem to be vehemently opposed to this. The reason that's mostly given is that it is "terrible writing", but how is it terrible writing?? I've never seen anyone explaining why that would be terrible writing.

Edit: People keep on mentioning disrespect but that still does not say anything at all. What is disrespectful about offing him off screen?

Edit2: Off screen means during the time skip where we would see it during a flashback.