Not comparable. Mrs. Kennedy did not have a duty station with defined authority and responsibility, she was just a passenger in a car. Mrs. Kennedy had not been trained her entire adult life in what to do in that stressful situation, as Scotty had with battle drills, casualty drills, damage control drills and so on. Scotty not only had those years of training behind him, but had himself commanded Enterprise in battle conditions and done very well. It's just not in his character to fall to pieces. At best it's a disservice to that character. You want to believe that Scotty, instead of doing what was best for the life of his nephew as well as the lives of the rest of the crew, simply lost his mind in that situation? I don't.
And Scotty had never been prepared for the possibility that his nephew would be fatally injured during an attack. That it happened in spite of his military training makes it all the more tragic--which happens to be the point of the entire movie. Meyer was trying to put the characters in positions they'd never faced before--no-win situations--to see how they would deal with them.
Scotty panicked and went to the bridge instead of sickbay.
Clark Terrell turned his phaser on himself rather than kill another Starfleet officer.
Spock chose to sacrifice himself to repair a damaged power conduit.
Kirk was prepared to rush headlong into a radiation-filled chamber to get to his friend until McCoy and Scotty held him back.
What Meyer was trying to demonstrate is that even the strongest individual, when faced with a situation that he believes is untenable, may react in a manner completely different than how he has reacted before--or he may act exactly as he's supposed to. It would seem that Spock's choice was appropriate but tragic.
One could argue that Terrell's choice was also appropriate given the circumstances--I don't understand why McCoy didn't take both he and Chekov back to the
Enterprise as soon as Chekov mentioned what Khan had done to them--but neither Scotty or Kirk seemed to behave in a manner that was appropriate, which only underscores the deep sense of grief and loss each was feeling.