I was very startled to see three half naked men running at me. Then they all turned into were bears and completely mauled me. (im playing on master).... is this normal?

you where playing on master so you obviously wanted a challenge well you got one.

Sounds less like a challenge and more of a, "What what are these guys do....AHHHH run, oh F*%$!, What just happened."
On Topic
I believe werebears are hostile when encountered, but I've yet to play DB so that's just what I've heard.

Yup. Its completely normal, all werebears are hostile and they'll start out as half naked guys but they'll change into their beast form. They are random encounters on the island, but there is a place they spawn regularly.