Is this behaviour dangerous?

in short, yes. despite that there are several potential reasons/interpretations of her behavior; it sounds like she is having a hard time with whatever is going on in her life at this particularly difficult time/age, and is lashing out at your friend. any child who repeatedly makes threats like that has some sort of issue, whatever it might be. there are several possibilities. i would suggest that katie stay away from kelsey until she can get a grip on her emotions and anger. she might benefit from some counseling because there is definitely something on her mind. she could just be projecting...but you don't want to risk the safety of your friend and her baby on a could be. i suggest getting her some help. there is no shame in that. the shame is in ignoring things and hoping they go away. pushing and threatening a pregnant woman, and a 10 yr old should know better, can not be ignored behavior. that is serious. but, don't be too judgmental with her, she just needs some help.