There's kill stealing, there's kill securing, and then there's "oh ****" kill steal.
Kill stealing is clearly when you just jump in and grab the absolutely gauranteed kill because you're a jerk. Don't deny it, most of us have done it. Even I've done it... I think...
There's kill securing where, despite the fact that your adc, or mid, or whatever could've gotten the kill, there's also a 50% chance the enemy could also get away. Better he dies and someone gets the gold, than he get away and no one gets anything.
Then there's "oh ****" kill stealing, which is the most common form of kill steal. This is most often where your support steals the kill from adc, or where the pentakill is stolen. It's in no way intentional, it's simply, you're contributing to the team fight and/or trying to innocently grab an assist(who wants to deny their teammates assists?) and just accidentally grab the kill. It's usually just a lucky(unlucky?) autoattack in most cases.
Admit it, most of the times you have ks'd, or been ks'd, have been the "oh ****" variety. Or occured in a team fight where it doesn't matter who got the kill, as long as you win the fight.