Keep in mind that a kill isn't always a kill. After all, one of the things that defines a carry is that they get more use out of gold than, say, a support does. If a pair of teams in a match are even on kills/assists, but on one side the support got all the kills, while the carry got the support on the other side, you can safely assume that odds are that in the endgame the second team (where the kills were scored by the carry) will be the one steamrolling.
And, of course, there's plenty of intent to killsteal in many games, where a player is often focused not on winning, but instead thinking their K number is the most important reflection of the ability. You often see this when an enemy champion is clearly not getting away, (even at low levels, you can tell if they've got two champions to run past and they're about dead they're not getting away) but will instead blow their ult so that THEY get the kill. Such a situation is not a "kill secured" in the least; it's downright wasteful: it denied plenty of gold to the champion that likely needed it more, as well as ensured that the team has one fewer ult at their disposal.
Granted, at lower levels an "accidental/unintentional killsteal" is pretty liable to happen; more often it's less by players not knowing the fleeing enemy has spent all their resources (Flash, CDs, etc.) but more by others not realizing that another player already was comboing into a finish, or didn't realize their attack would finish them off. (A classic example would be Lux's Final Spark)
However, there's a major difference in these sort of unintentional cases: a player who unintentionally stole a kill will quickly apologize, often of their own volition. Someone who intentionally stole it? They won't apologize, and if the KS is mentioned will often quickly become toxic in behavior. (if they weren't already) This sort of behavior will, in fact, definitely hurt the team's ability to work together.