Have you ever outplayed, out-smarted, and out-gunned the enemy champion in your lane, and just when you are about to close in for the kill with that final perfectly timed attack, a teammate of yours that happened to be lurking nearby swoops in and steals the kill? I bet you have and you know how irritating that can be. (Ambien) To do all the hard work and then have someone else steal all the glory right before your eyes. This is called Kill stealing or KS-ing for short.
This is a phenomenon not just in League of Legends but in other games as well. In some games, the experience and gold or in-game currency you earn depends on the damage you did and not whether not the final killing shot was by you. However, in League of Legends, all of the gold goes to the killer and that can make it frustrating. Good team-mates will not steal your kill but if you have a toxic cry-baby among your ranks, you can expect a lot more KS-ing.
Sometimes, during team fights, it’s often not clear who is going to get the final kill when everybody on your team is trying to kill everyone on the enemy team. KS-ing can be forgiven in such scenarios. When you are chasing down an enemy and he seems to be getting away from you, a teammate coming to ensure the kill should not really be considered KS-ing. At times, the support to the AD-carry sets up the kill but ends up taking the kill. If that happens only once in a while, it’s not KS-ing.
However, if the support is continuously trying to take your kills if someone from your team withholds using his high-damage abilities and uses them only when the enemy is at very low heath, that is the textbook definition of KS-ing.