A teen who fatally stabbed a WABC Radio newsman he met over the Internet and agreed to hook up with for sex was found guilty of murder yesterday by a Brooklyn jury.

But even the murder conviction couldn’t wipe the grin off the face of John Katehis, now 18, who faces 25 years to life in prison after stabbing George Weber 50 times in March 2009 inside Weber’s Carroll Gardens apartment during a Craigslist hookup.

Katehis claimed he was protecting himself after Weber, 47, whipped out a knife during sex play.

The smirking Katehis, whose first trial ended in a hung jury last year, sat motionless as the jury forewoman announced the verdict.

But as he stood to be handcuffed, he turned to his father sitting in the second row, shrugged and broke into a wide grin.

“This is the demeanor of a cold blooded killer,’’ prosecutor Anna-Sigga Nicolazzi had told jurors in her summation Monday.