Best Friend ���Ruined My Life��� When He Killed His Own Parents

Michael Mandell, whose best friend, Tyler Hadley, is in prison for killing his own parents before throwing a house party, said his life was ruined because of the murders.

It ruined my life. It ruined Tyler's life. It ended his parents' life, Mandell, 20, told ABC News 20/20.

I can't go to school without people saying, That's Michael Mandell, and relating me to something this sick. And that's what everyone knows me as ... the best friend of a murderer.

On July 16, 2011, Tyler Hadley, then 17, bludgeoned his parents, Blake and Mary Jo Hadley, to death with a hammer at their home in Port Saint Lucie, Fla. After cleaning up the blood, he then had a house party that night, while his parents bodies remained in their bedroom.

Watch the full story on ABC News 20/20 tonight at 10 ET

Mandell grew up just a block away from Tyler Hadley. I was just walking around, looking for friends, and I met Tyler He slowly became my best friend, Mandell recalled.

When Hadley was 15, his family said they began to see a change in him.

He started skipping school, hanging out with the wrong crowd, getting on drugs, Tyler Hadleys aunt, Cindy Hadley, told 20/20. We knew marijuana.

By the time their son was 17, Blake and Mary Jo Hadley had tried everything to get their son on track, including a psychiatrist, antidepressants and an outpatient mental health and substance abuse program, but nothing changed.

After Tyler Hadley came home drunk one night, his parents took away his phone and car. He felt that, I guess, that was too much, and that she was over disciplining him. And he said he wanted to kill her, Mandell said.

To Mandell, it didnt seem like Hadley would actually do it. It sounded like he was mad he was getting punished, he said.

Just a week before the party, Tyler Hadleys parents took him on a family reunion to a cabin in Georgia. I never saw Tyler lose his temper, ever, Cindy Hadley said. That's what is so bizarre about everything.

The day of the murders, Hadley posted an invitation on Facebook to a party at his house. Mandell said Hadley told him that his parents were away in Orlando, Fla.

Young people [were] drinking and sitting around talking. But, the weird thing was, most people in there, Tyler didn't even know, Mandell said of the party.

READ MORE: Teen Took Selfie With Best Friend After Learning the Friend Was a Murderer

During the party, Tyler Hadley pulled Mandell aside and told him he needed to tell him something, so the two went outside to talk.

He says, Mike, I killed my parents. And I said, No you didn't, Tyler. Shut up. What are you talking about?" Mandell said. And, he said, Mike ... look at the driveway, all the cars are there. My parents arent in Orlando. I killed my parents.'

Mandell said he went inside and saw blood by a computer desk and more blood by the master bedroom door, but he thought Hadley was playing a prank on him and still didnt believe he killed his own parents.

I took Tyler into the garage, and I said, What's going on here? I want to see them dead, if you really did it. I don't believe you, Mandell said.

He said Hadley told him the party needed to die down before he could show him the proof, but Mandell refused to wait and went to check for himself.

I come up to the [master bedroom] door. The party's going on over here, and I turn the door knob, Mandell said. I looked down, and I [saw] his father's leg against the door My eyes popped up, and I said, Oh my, he's telling the truth. He did it. This is real.

Although he discovered that his best friend was a murderer, Mandell said he stayed at the party for a few more hours.

If you were in my shoes, and that guy was your best friend, you wouldn't want [to] leave right away, Mandell said. I know how heinous and sick this is, you know, what he did. But, you ... wouldn't have ran away, because you're comfortable with this guy. You don't see him as a killer.

Mandell even took a selfie with Hadley, because, he said I knew it was going to be the last time I ever [saw] him. But even while posing for that picture he knew what he had to do.He left the party in the early hours of the morning and called Crime Stoppers to tell them Hadley killed his parents.

You know, I didn't have any hesitation whatsoever for calling the cops, Mandell said. The worst feeling had to have been realizing ...that his parents are gone.

At a sentencing hearing in March Hadley, now 20, broke his silence but offered little insight into why he killed his parents.

"Not a single day goes by that I don't think about my parents or my whole family that I've affected by this, Hadley said. I just want everyone to know that I am ... truly sorry for, you know, the acts I committed.

Tyler Hadley was sentenced to two life sentences without the possibility of parole for the murders of his parents.

It makes me realize ... you never really know anyone, Mandell said.

And since this, you know, I have trust issues. And I feel like I'll probably, you know, never fully trust any new stranger that I meet again.

Tune into ABC News 20/20 tonight at 10 ET to find out what message Michael Mandell has for his best friend Tyler Hadley.