Let us begin with the beginning of Larry's life, and a pregnant 15 year old Lily Evans.
Larry was conceived on a Christmas night. Romantic, isn't it? Lily and James were the only Gryffindors who chose not to go home, and boy, was James thankful!
Nine months later Larry was born.
Larry grew and grew, and he was kept in hiding. No one could find out about him. When he was born, Lily and James were immature teenagers. It was going to be better now. The next child would be okay.
How would you feel if you were ignored by your parents who liked to pretend you didn't exist?
I can assure you, Larry didn't feel great either, and it didn't get any better when he was sorted into Slytherin, the only one knowing he was a Potter being Dumbledore. You see, Larry went by the name of Tom Carter to protect the future of his future sibling and his parent's name. However, Larry became popular and had a big group of devoted friends. He started to become the dictator of his own life, and at age twelve he decided to move in with a family he had become close to while at Hogwarts: the Riddles.
The Riddles were a pureblood family who empathized with Larry, who was now being called Tom by everyone, even by his parents. They taught him to use his pureblood genes to get approximate to power, they taught him about everything. So Tom became a strong wizard, and a dark one too. Everyone, including people he had once been close with, were fighting against him. His parents were leaders of this fight, as they didn't want their dirty, little secret to bleed out. He had to eliminate them.
On October 31, 1981, Tom did just that, he eliminated his parents. They now had a little son named Harry. Tom tried to kill him, too. But it didn't work.