I need advice, (kids talk about killing others) help me with this

Hi my name is Christina and I have a stepson that I raised with ADHD from the age of 3 to 24 even throw my divorce he stayed with me. I also have a son of my own that is 18 getting ready to graduate, he's mature and we have a great relationship with respect and close enough that he knows he can come to me for anything. Both of them know they can come to me and I can talk to them and understand them and then give them good advice in a way, that shows them that even though I am the parent I will help them understand in a way easiest for them to come to the correct decisions. Now putting this aside, I am dating a man that has three girls and we all get along good. But I noticed something in his youngest one that has me a little worried. I read kids pretty good and can talk to them with no problem. They seem to trust me too.
The other day my boyfriend and I and his girls went on a trip and the youngest one that is 8 and in second grade (I will refer to her as Ashley) I know Ashley likes me but every once in awhile she comes out with things that are uncharacteristic for any 8 year old. For a tiny example we were shopping for Ashley and I helped her pick out her clothes and I made sure she was happy with the choices we made, and her not just saying okay because I know I never liked the clothes my mom bought me and wouldn't wear them. So I was making she was sure that she was pleased. When under her breath she said "why do you have to keep talking?" and she said it where she wanted me to hear it, Which I understand how kids think we talk to much but I wasn't, I was giving her the option of making her happy. So I found it odd and a bit rude but let it go. When you look at her she's cute as a button but there is also something deeper going on, trust me on this. I know she likes being close to me and even though she likes me and acts like a sweet angel at time you always know there is something she thinking. Because she comes out with some odd comments. Later I told my boyfriend (refer to as Joseph)about it and we were going to talk to her about some of the things that I've noticed. But we were on vacation visiting his family (Mother, and his brothers family which he has 3 girls of his own).

Now the next day we Joseph & I were out while his three girls were visiting with his family and when we got back his mother told us the Ashley made her cry with something said to her and she also acted and upset his brothers kids as well. He tried to make her apologize but she wouldn't. When we left the Josephs mother told him that she didn't like the way Ashley behavior was at all.

We decided to deal with this at home. When returned home Josephs kids went back to their mother (refer to her as Cathy)house. The next morning Cathy called asking Joseph to watch Ashley because she was sick. So no problem but when Ashley was there she was a little sick and spent some time drawing and they had a long talk about what issues that came up over the trip, she said she understood and that she was sorry and she would work on it. Later I came over and Ashley had returned to Cathy's and I needed a piece of paper to write something down Joseph handed me a tablet that the eight year old had used to draw when I was flipping throw the pages to find a clean page I ran across this

" THIS IS A KNIFE (drawn picture after word) IT WILL CUT YOU___________ MOVE ON..... THIS IS A GUN( drawn picture after word) IT WILL KILL YOU____________ THATS IT"

I couldn't believe my eyes I showed him he then called Cathy which then she told us she wasn't surprised because on this last New Years Eve Ashley's resolution she share with the family she was going to kill them all in the night. Also Cathy has a little girl from her second husband that when the Ashley gets made at her little sister she tries strangling her. So the Cathy said she is now in counseling. Now we all know this is not normal behavior from a eight year old. I just don't know if its because of something that can occur because of something else or is there something wrong with her mentally.

Thanks
Sincerely Yours
Christina