In Grimm when Juliette becomes a hexenbiest why does she become evil?

Because thats what a hexenbiest does to you & she didn’t know how to control her hexenbiest powers. Adalind was the same when she had her powers

It's the 'nature of the beast', she had to overcome that to be good again.

I think there's a couple of reasons:

1: Anger. Anger at Nick for bringing her into his world with Wessen and Aadilind, etc.

2: Power corrupts. Juliette suddenly gained the full power of a Hexenbiest but none of the training and upbringing to temper that power.

It's also why everyone is so worried about Diana. Should she be swayed towards evil and darkness, she could do a _LOT_ of damage.

3: Aadilind mentions that Hexenbiests aren't particularly nice. So Juliette's behavior may just be natural.