Kimi screwed up went wide then cut back making a big error in that he presumed Vettel would go very tight and there was no one on his inside. Unless everyone from 2nd back has a truly awful start then if you screw up and go wide in the first corner a couple of guys will take the inside line from you. Kimi should have continued the corner in a very non aggressive manor, assuming there is probably someone on the inside rather than no one there. Had he done so and stayed wide he'd still have got a good exit but given everyone time to react including himself.
Kimi instead cut very aggressively into the corner after going wide and went right across Vettel. Vettel was not on the apex, he was obviously a full cars width from the apex, he's essentially on the middle line through the corner, Kimi tried to get on the middle line of the corner despite Vettel being there. He assumed Vettel would turn tighter and onto the inside line, Kimi went onto Vettel's line, it was completely Kimi's fault.
I'm absolutely fed up with people not making allowances for the first corner. You screw up the line you can't aggressively cut back, it's awful driving and 90% of the time someone does that on the first lap it results in contact. Any other lap you know who is within range of making a move but on the first lap and first corner you should absolutely assume that if you screw up there will be 1-2 people able to take advantage and you should then assume there is an extra car or two around you as you recover from whatever mistake you made.