Natalee Holloway's Brother Says He'll Hurt Joran van der Sloot if He Walks Free

Natalee Holloway's brother has serious doubts Joran van der Sloot has found God after killing his sister ... and he's ready to hand the murdering monster a beatdown if he ever walks free.

Matt Holloway has been a correctional officer in the past, and while he tells us his experience is that many inmates turn to religion ... he also thinks van der Sloot is lying through his teeth about making that change.

He thinks van der Sloot was only honest about Natalee's death for selfish reasons -- the fact is, he struck a plea deal -- and Matt promises he will "whoop his ass" the day the killer is a free man.

That could happen in about 22 years. Van der Sloot got a 20-year sentence last week in Alabama when he pled guilty to extortion and wire fraud and confessed to killing Natalee. However, he'll serve that sentence concurrent to the 22 years he has left to serve in Peru for murdering a woman and drug trafficking down there.

Right now his scheduled release date is June 2045, when he'll be 57.

We also talked with Matt about why he thought it was practically impossible to nail van der Sloot for Natalee's killing all those years ago -- and he feels van der Sloot's father being a high-profile attorney in Aruba interfered in the investigation.

Matt says he's certain corruption was at play in van der Sloot dodging authorities for over a decade ... and he's calling out Aruba for, in his eyes, purposely dropping the ball. He is also calling for the Aruban government to look into any potential charges it could bring against van der Sloot now that he's copped to killing Natalee and misleading police.

In his chilling confession, van der Sloot said he took a cinder block and smashed Natalee's head in on the beach in 2005 -- this after she denied his sexual advances by kneeing him in the groin.