@Background Pony #9BB4
No she wasn’t, a sociopath, simply put, is a person who cannot empathize with others. This means they have the capacity to be exceedingly cruel without any regret as they cannot feel it.
Lightning Dust is a maverick, a pony who knows what she wants and will go 110 percent to get it. She wanted to be the best beyond even Rainbow’s ambition. She never believed any of the people were in danger, as when she got to them, the problem was already solved.

Honestly, if a random group of ponies got in an accident, and Rainbow saved them, do you think Rainbow would be so sensitive when they aren’t her friends? She would say, ‘Yeah, I’m awesome’, or something. Despite the fact these ponies nearly died, she would be, ‘cool’ about it.

@Background Pony #9BB4
Oh yeah? Remember when Rainbow accidentally destroyed the dam in Mysterious Mare Do Well and Twilight had to fix it? Rainbow didn’t care, all she cared about was that somepony was better at something than her.
You are bringing up a problem with Wonderbolt Academy, in which Lightning Dust gets blamed at the end for all the things she did that Rainbow did too. Rainbow is at fault for clipping her wing, she blew half the cadets away while overtaking them, and she helped create that tornado. Rainbow helped create the tornado that nearly killed her friends. She didn’t take responsibility, she just blamed Lightning Dust. And Spitfire does an unfair 180 on her by siding with Rainbow when she convinces her.
That episode, while fun, is a example of a character not taking responsibility for their mistakes and implies that if you do something wrong, it isn’t your fault if you were told to do it by someone else.
And you can’t say she is a sociopath, for 2 reasons. The first is that we saw very little of her and to form a prognosis with that is silly as MANY people would be seen as sociopaths on their bad days. The second problem is that Lightning Dust and Rainbow are friends during the episode by Lightning’s introduction. Sociopaths don’t have friends, they get befriended and then use people. If you’re about to think, ‘well, Lightning Dust used Rainbow’, remember they are flying partners for goodness sake, and Lightning NEVER forced Rainbow to do anything.
Furthermore, if you want to go by Expanded Universe, Lightning Dust is friends with Rainbow after the episode, apparently, because during the short novel for Fluttershy, Rainbow is hanging out with Lightning at a fair.