This sounds like the Strange Forces series by Marty M. Engle, which was a spin-off of the Strange Matter series. Hope this is what you're looking for!
The students of Fairfield Junior High have noticed that the strange things happening in their town are becoming more common — more dangerous. With each dreadful encounter, they are further convinced — monsters are real.Morgan Taylor is convinced. He couldn't deny that a small lizard-like monster was rapping on his bedroom window, asking for help. A real monster, just outside his window.Astonished, he lets it in. With wide eyes he listens to its amazing tale and desperate plea for help — but could he believe the little monster's story? A tale of an object so incredible it staggers the imagination and of the hideous man-creature, the Collector, who has come to Fairfield to claim it.Dark forces have gathered under the Collector's command, in a hilltop mansion on the edge of town — Fairfield Manor. The crumbling halls, once empty and silent, now echo with the howls of his army of creatures. They have filled the once-barren rooms with mysterious artifacts gathered from the four corners of the earth. Nothing will stop the acquisition of the Collector's greatest prize.Join the students of Fairfield Junior High and the renegade lizard-monster, Rilo Buru, in a race against the Collector and his strange forces on an adventure that will change the natural and the unnatural world forever.
This review of the second book in the series mentions them being able to transform:
This starts a bit after the last one as the group of kids are still hanging out with Rilo the Buru. See, during the first one he gave them these special powers that let them transform into various creatures so he is helping them train to hone these powers, but under the hopes they wouldn't need to use them in a fight again.
Darren Donaldson is the one who turns into a werewolf, as per the summary of characters at the opening of the first book:
![enter image description here](https://cdn.statically.io/img/i.sstatic.net/yrM4HBl0.png)
The TV Tropes entry mentions a character who can cause accidents, albeit not among the main cast.
Jerry, one of Guerendet's henchman, has the ability to cause severe and often fatal misfortune to befall anyone he chooses. He started off as a simple accountant in New York who never did anything unusual until he realized his powers when he caused a coworker to have an accident.