I generally loathe the practice as I'm a genuine team player by nature, but can appreciate the fact it's necessary in certain situations. The general unspoken rule of Gears is basically any down within walking distance is off limits, but any far beyond that is naturally forfeit. The rule receives a minor tweak for game types lacking Execution rules, but it's more or less concrete. Problem is a large majority of stolen kills in this game are deliberate acts of being a class S douche. More often than not the perps already lack the necessary skill to contribute in meaningful ways thus discrediting any sort of "for the team" bullshit logic. They aren't interested in winning that much. They usually do it to band-aid their wounded k/d to appear competent undermining the incentive of teamwork while reinforcing the same lone wolf sentiment that sabotages what could have been a guaranteed win.
Better players already have the burden of carrying them for the entire match. Why make matters worse for them by deliberately stealing their kills? It's more of a common courtesy/respect issue than about the kill IMO. Let that guy who earned the down get his just due if he's in the clear to get it, you know? Don't just walk up or take something that isn't yours after you've been revived then use flawed logic to mask you being a jackass.
I'm not criticizing people who do it by accident or kill someone who is about to get revived. It's common sense to kill them. I'm criticizing the scrubs who join games JUST to piss people off by doing it as much as humanly possible.