Is Kill Stealing a Thing?

I'm fairly new (level 14 right now), but I've been playing a lot of Ziggs and having tons of fun. Ziggs has all AoE skills, and when I see an ally being ganked I'll usually drop my R on the enemies while they're distracted. However in a match recently one player was telling me to stop 'stealing kills' and to not try 'going for the kill' at all. This is confusing to me, because as I see it... 1. I can get a decent number of kills without feeding, so why not? 2. We both get experience and gold if the enemy champion/s die to me or my ally. 3. Your 1v2 might not go as planned. Is accidental kill stealing actually heinous or was this just a salty dude only interested in having the most kills? I wasn't robbing this guy blind, he had a few more kills than me, and I had a ton of assists from helping out teammates.
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