The real reason Ridley killed Samus' parents: Ridley's race's mating habits are strange; violence, hatred and bloodshed is how they express themselves (they do not understand the concept of ****). Unfortunately, this has resulted in the near extinction of their kind.
Ridley, not wanting his species to die out, sees the opportunity to have Samus grow up to hate him and continuously try to kill him (it's easier to "prepare" children for their future mating ritual than it is for adults for the moment). While Samus is obviously a different species, Ridley's choices are heavily limited due to no other known member of his race, let alone a female.
Ridley's plan succeeded too well, as it resulted in his death on Zebes. Though in a way, he did continue the lifespan of his species, as DNA from Samus' suit from their final encounter was accidentally used to clone a new Ridley. ....Unfortunately the clone developed too fast due to unstable genetics and never really grew out of the "kill and devour" stage, though it grew some undertones of mating, as shown through his teenage state. Even more unfortunate is that the clone chose to pursue the very same being that killed his predecessor, and it resulted in severe injuries that cost him his short life because of an unexpected encounter with a Queen Metroid.
I swear, I have potential to be a good writer if I wasn't so frigging lazy to work on my literary skills.