And of course, in Ellison's original draft, it is Spock who prevents Keeler from being saved, not Kirk, who was supposed to be the series leader and man of action.
The heck with the man-of-action stuff -- what makes the aired version better is that it's a more wrenching choice for Kirk to make. If Kirk tries to save Edith and Spock stops him, as in Ellison's version, then each is just following a simple, linear path, making the choice they were already inclined to make. But what we got is much more powerful, because Kirk resisted his personal feelings to make the harder choice, and has to live with the guilt and pain of that decision.
And it's a better arc for the relationship between Kirk and Spock, because Kirk listens to his friend and has his mind changed, rather than just following his own impulses and ignoring Spock's warnings. And Spock's line at the end -- "He knows, Doctor. He knows" -- shows his sympathy for what Kirk is going through.