Why did Voldemort kill his parents

Why did Voldemort have to kill Harry Potter's parents?

Because he had to kill Harry and they were in the way. Some of this is explained in the books.

Voldemort had no qualms about killing people when they became obstacles. When James tried to protect his family, Voldemort eliminated him. Lily is a different matter because Voldemort didn't want to kill her at first (at Snape's request). However, when he was there, and after killing James and trying to kill Harry, he thought it was best to just kill "the whole set":

He could have forced them away from the crib, but it seemed wiser to take them all out ... - ( Harry Potter and the Deathly Hallows )

Nor did it help Harry's parents that they were in the Order of the Phoenix, which actively fought against Voldemort and his followers (remember, the prophecy that sparked all of these mentions that the child's parents "defied Voldemort three times") . It is possible that he could be her too without that prophecy would have killed.

In short, Voldemort was a murderous villain with no conscience, Harry's parents were Voldemort's enemies, he came to their house to kill their son and they tried to defy him. It's not so surprising that he got rid of them.

Deepak

Was Snape previously a friend of Voldemort's?

Walt

Snape was a Death Eater and served Voldemort before Harry was born.