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 intend to kill her at first (at Snape's request). However, once he was there and after he killed James and was about to kill Harry, he thought it's best to just kill 'the whole set':

He could have forced her away from the crib, but it seemed more prudent to finish them all... --(Harry Potter and the Deathly Hallows)

It also didn't help Harry's parents that they were in the Order of the Phoenix which actively fought against Voldemort and his followers (remember that the prophecy that triggered all of this mentions that the child's parents have "thrice defied" Voldemort). It's possible he would've killed them even without that prophecy.

So long story 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 that surprising that he did away with them.

Well in the prophecy, it was either Harry or actually Neville Longbottom. Neville could've had the scar if Voldemort chose to go to his house when he was a baby when he was younger, but he chose Harry instead. Both harry and Neville were born near the end of June and their parents were in the Order of the Phoenix, the reason why Voldemort chose Harry's parents is indeed unknown, but it could've been either way.

But that still doesnt say why he needed to kill Harry a baby. I think there was a rumor that Harry was the savior born to rid the magical world of evil i.e. Voldemort. I think the prophecy was Harry would be strong enough to get rid of him but to me if his parents were just regular wizards why was Harry so special? I think that it was just a rumor that he was the chosen one and that Voldemort made Harry strong enough by mistakenly making him a Horcrux and in turn making the rumored prophecy true. Now please know that I have not read the books only watched the movies so Im not as well informed as the readers. Just my interpretation.