Given Peter Parker's incredible intellect, wouldn't Spider-Man be smart enough to know sling-shotting Gwen would break her neck?
closed as not constructive by NikolaiDante, Micah, John O, phantom42, K-H-W Feb 7 '13 at 2:02
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Spider-man is superhumanly strong and tough. His body recovers from injuries very quickly, he can shake off blows from people who would turn a normal person into a fine red mist, and he can lift a truck.
He's also learned almost everything he knows about webslinging by instinct and experience.
Thus, his initial experience in webslinging (and recall, he was still a teen when Gwen died, not the experienced adult he later became) is with the effects upon his own body.
Suffice to say, he can take far more G-Forces and far more punishment than most humans. Had he stopped himself from falling the way he stopped Gwen, he would have survived with little more than some residual bruising and soreness. In fact, he'd stopped himself that way previously.
Time was critical - he had literally a split second to choose a course of action and act - he reacted on instinct. His knowledge of physics of normal human bodies requires thought.
In short - he made the worst possible call for the best reasons.