Hi, I'd like to help you with your question.
It sounds like your son's girlfriend could have either some underlying anger issues or she is just nervous around you. If she has anger issues, most likely she was exposed to something violent or something that created anger in her as a child. For example, if she witnessed domestic violence or had relatives that had anger issues. She could also have been abused but I think you would be seeing more symptoms than just inappropriate statements if that were true.
There is a big difference between anger issues and someone who is dangerous.
Has your son ever mentioned his girlfriend acting out towards him such as hitting him or hurting him "accidentally"? Or has he witnessed her harming anyone else? This would indicate that her problem is more than just anger related.
Has she ever hurt an animal either seemingly accidentally or on purpose? Does she play mean games or tricks on people and think it is amusing? Does she seem to take delight in seeing people get hurt? Does she seem narcissistic or does she like risky behaviors? Traits like these could indicate someone who is sociopathic or an anti social personality disorder.
If your son's girlfriend does not fit any of these descriptions, then she most likely is just either uncomfortable around you or she does have some mild anger issues.
Either way, keep in touch with your son about it. You may not want to let him know you are suspicious of her behavior but instead that you are interested in the relationship working out for him. And then keep an eye on the situation. Most likely, it's just a minor problem she'll outgrow as she matures.
Let me know if I can help in any other way,