A kill steal would be you killing someone that had no conceivable way of escaping the person killing them, as well as having no chance at killing your ally.
Then I witnessed, far too often, a situation in which there is no way the enemy is going to get away from this ally, so I back off, and then BAM, he hits a trap, or the runner got that one skill they needed off cooldown, and gets away.
I've since stopped caring about KSing.
KILL THEM! IT DOESN'T MATTER WHO MAKES THE LAST BLOW, JUST KILL THEM!