I'm looking for a sci fi book I read once, sometime in the last 10-12 years. Probably published around that time, too It was an ebook, possibly/probably on Baen. Although every time I've searched since, I've failed to find it.
The premise was that a section of humanity got superpowers overnight. To begin with they were welcomed as superheroes but...as might be expected, they turned criminal, robbing banks and carving out their own empires. Possibly called Ultras, they were super villians instead.
Each had a single weakness, and the book was about a group of people who tried to kill as many as possible of the ultras, identifying the weaknesses one by one. One member of the group lost his father in a bank robbery orchestrated by one of the ultras at the start of the book / when they first occurred.
It was widely believed that he was the sole survivor of the event and could identify that particular villains weakness. They even tunneled into the ruins of the bank to look for clues, and a weapon that once injured the villain (a pistol) - nothing special about it. It turns out that he could only be killed or injured if the person doing so had no ill intent towards him. He got tricked into firing the gun from previously with a detonator in the barrel and blowing himself up. It also turned out that a couple of people in the group were themselves super powered. Set in the US in the near future.