Coworker talking violently.

So this particular coworker I work with, I honestly don’t like her, but I tolerate her and don’t say anything to her unless I need to. Today 2 coworkers of mine said she said “I wanted to knock her head off” while conversing with the manager on open floors. The manger laughed. This isnt the first time she’s said something like this. One girl who used to work with us, she said she wanted to punch her in her face and and she called her a b**** in group Snapchat. She says all this in front of the manager, so the manager be aware of the way she talks. But he don’t do anything. He laughs or usually comments on it. She gets along very well with the manager and they talks about coworkers and even the assistant manger together. So he’s just as guilty of all this just as much as she is in my eyes. When they do talk, they talk on open floors omg don’t of customers, so they hear. This coworker, nobody really likes and she really don’t do much to be talking the way she does about other coworkers especially about putting her hands on them. I’m 20 and she’s 28 so she thinks she can talk like this to people. I honestly have done nothing to her for her to treat me the way she do, nobody does nothing to her. I’m thinking about talking to the asistant manger about her and the head manager and hoe they’re always saying negative things. Is there anything else I can do? I feel so uncomfortable working around them two.