How does a single star going supernova destroy an entire galaxy?

Cos Abrams is a cunt who doesn't understand space.

It didn't destroy an entire galaxy, it just destroyed the solar system where Romulus and Remus were.

Star Trek Online actually explained it far better than JJ ever did. The writers for the game came up with a brilliant explanation as to why the supernova was able to destroy a solar system several light years away within just a few hours. The explosion was not natural, and was traveling partially through subspace, moving far faster than anyone anticipated. Had this explosion been natural, the shock-waves would not have reached the Romulus System for 300 years, giving the inhabitants plenty of time to leave.

Evidently an ancient, dangerous alien race called the Iconians had teamed up with a rogue branch of the Tal Shiar and had developed a weapon using highly advanced technology, which was fired into the star to make it blow up.

Your character ends up investigating the Hobus Supernova, and discovers that time travel and Empress Sela were involved in pissing off the Iconians in the distant past, leading to the Hobus Supernova and the Iconian War a quarter of a million years later. It was far better writing than Abrams could even dream of.

Remember, in Abrams "Force Awakens" he had people on another planet in a different solar system able to watch other planets be destroyed as they look up in the sky with their naked eyes. He doesn't care about science or logic, just what looks good to him image wise.

Comic book physics and an inability to understand the inverse square law. It would be more believable if it was a gamma ray burst which is directional vs a supernova.

You do realize you're discussing astrophysics about a Star Trek movie, right?. Like, a supernova destroying a galaxy would be the most realistic thing in it.