Steal My Kill

I checked your mastery list and you have level 5 Vayne.

GOTTEM!

I've been playing support more recently, and get where you're coming from. In the situations described, that's perfectly fine. But sometimes, people go to unnecessary extends to get the kills (especially with the carry supports). This is not exclusive to supports taking kills when it's not needed. ADCs also do it too, flash+aaing a target about to die from the support's ignite. The difference is, ADCs (especially draven) need the kills more.

In teamfights, I also understand it's not intentional in most cases, but sometimes, people just screw around and kill targets mid-dunk.

Outside of teamfights, they'll take the kills in 2v2 skirmishes (between laner and jgl, or w/ the bot lane).

Situation that happened with me yesterday: You're 1v2ing the enemy laner and the jgler, and are about to ult to get the 5 stack steroid by ulting the enemy jgler. Mid-fight, the jgling Khazix, who wasn't even IN the skirmish earlier (hence the 1v2) E+Qs the jgler to get the kill and then runs off, as I q the enemy laner, but without getting 5 stacks (because my ult was cancelled).

Thus, I die to my laner and miss a double, because of the Zix taking the kill.

I always get pissed when people can't read between a kill that can be left alone to be secured and the priority of having the target die over who gets the kill. For example, if you go ham early bot lane and the target is one hit away from death, I hate it when supports back off trying to "feed" me the kills when they're close enough to get it. Having a bot laner die is the same as me getting the kill due to the free farm, free of harass and pressure I get.

I always tell my supports to go for the kills. I only ever get upset when it's an obvious secured kill (we KNOW they have 0 summs and no help on the way).