The massacre is not (yet) explained anywhere else either.
Most likely explanation would be that for anyone not of the Skywalker line, becoming a Jedi is a decades-long process (judging by the prequels where Anakin is stated to have become the youngest Jedi Master ever at around 20, even though other Jedi had been learning the arts since infancy), whereas Skywalker get the hang of it much faster (as apparent by Luke's progress in the four years in-universe between ANH and ROTJ), so BenKylo got a significant headstart on the other students. The massacre still required that Luke was elsewhere (a distraction arranged by Snoke, no doubt), or else he would have wiped the floor with Ben.
Yes, pain may make Dark Side users stronger, but only to a point, not where you take the equivalent of a grenade launcher shot to your guts. And Kylo is still unsure of himself and of his darkness, no matter what he boasts to Snoke - he would not have been able to kill his father without Han's unwitting help. Also, Kylo never was fully trained, sure he can intimidate non-Force users, but he's far, far from being a fully learned Jedi or Sith. Probably he has not learned much (if at all) since killing the Academy, and lightsabre-armed opponents are not exactly common in the galaxy far far away's present ("an elegant weapon of a more civilized age"). He seems also very immature for his age (intended to be about 30), the way how he behaves and having all these temper tantrums and wrecking things.
It is still heavily implied though that Rey did have prior training, and/or is either Kylo's sister or (most likely) cousin. Though, if she did not have training, that would explain the title of the film, that the Force is literally awakening and has selected her as the new "Chosen One".
Also, on second viewing, the moment Kylo is told that "a girl" helped BB-8 escape, his reaction clearly indicates that he knows or suspects who she is, and in the interrogation scene clearly behaves as if he knew her (though she seems to have forgotten him). Now, if these were the prequels as written and directed by Lucas, this could be just another unintended accident (cf. the hamfisted scene where Padmé questions Anakin about Jedi code, and Anakin's explanation, acted and written as if he's bending the rules to suit him, is actually the official Jedi policy as per Lucas's interview afterwards), but I don't think so this time.
