He saw something, so why the hell didn’t he say something?

When the man known as “M.B.” noticed a Web cam pointed directly at the Rutgers dorm bed where he was doing the deed with his 18-year-old freshman boyfriend, Tyler Clementi, it should have set off alarm bells from there to the Hudson.

Come on, people. In an era when anyone with a Wi-Fi connection feels the need to tweet, Facebook or text everything they see or do, M.B. should have stopped and smelled the spying.

Did he freak? Warn Tyler? Yell at the top of his calm, collected lungs?

None of the above.

“I just thought it was strange,” M.B. testified in a monotone that’s glaringly annoying in light of everything that happened since.

A day after the intrusion into Tyler’s private space, the young man killed himself by jumping off the George Washington Bridge and plunging eternally into the river.

“Strange,” M.B. called it. Are you freakin’ kidding me?

“Being in a compromising position . . . it just caught my eye that there was a camera lens looking right at me,” he muttered.

But if the sight of someone spying upset him, M.B. soon forgot, or he just didn’t care. He never said a word to Tyler.

M.B. — he gets to keep his privacy — was the creepiest star witness in this sad and strange trial that has transfixed New Jersey. Tyler’s roommate, Dharun Ravi, is charged with invasion of privacy and bias intimidation — a hate crime that could cost Ravi 10 years of his life behind bars.

But Ravi is not charged in Tyler’s death. Which makes folks around here wonder: What killed Tyler Clementi?

M.B. slumped into the courtroom, tall in a blue shirt and thinning hair, speaking calmy and quietly about a death that angered the nation.

Tyler had just come out as gay to his parents (his mother, he told a friend, rejected him) and met M.B. on a Web site where lonely gay men meet and greet. The pair hit it off — under the sheets.

M.B. and Tyler had three trysts in September 2010. Though M.B. was shy about sharing details, making their friendship sound like a trip to the library, prosecutor Julia McClure got the dirt out of him.

“Did you and Tyler Clementi engage in sexual relations?” He reluctantly said, “Yes.”

“Were you naked in those sexual relations?’’

“Yes.’’

And the last question made me wonder how anyone who even remotely liked Tyler, described as sweet and shy by all who knew him, could ignore the elephantine camera in the room.

“Did those sexual relations involve sexual contact and penetration?”

The answer was “yes.” M.B. seemed mighty relieved when the subject changed to what a rotten boyfriend he was.

M.B. was hammered, for nearly three hours, by defense lawyer Steve Altman over the fact that he never took Tyler for a meal, to a movie. For a crummy cup of coffee.

His own lawyer, Richard Pompelio, said Tyler had cut him out, and M.B. heard the news of his death on the radio. When investigators went to see him in early October, M.B. hadn’t talked to, texted or laid eyes on his close and intimate buddy in 10 days.

It may never be known what pushed Tyler over the edge — rejection by his mother? M.B.’s indifference? Ravi’s disgusting betrayal? But one thing is clear:

Had M.B. sounded the alarm bells and warned Tyler that he was being spied upon, he could have protected his new pal from a roommate who called in friends for a “viewing party” in which Tyler was the main course.

At least, he would have shown he cared.

Instead, he was more concerned that Ravi’s friend’s appeared to be talking about him, making him feel “uncomfortable.”

Once again, M.B. said nothing about it. Silence, in this case, may equal death.