The incident happened halfway through to the first turn and you won't expect a racing driver to break and back off. If it was around 10 metres from turn 1, yes. But definitely not at midway through. There were 2 drivers who had nothing to lose and one that had everything to lose. Who do you expect to play it cautiously?
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What Vettel did was nothing crazy or too risky...under normal conditions. But Kimi was there and Vettel was not aware of his presence so a quite standard "close the door" maneuver became risky and finally a pretty bad move.