What a disturbing story Maria...
For a child that young to have behaviors that malicious --

I have a hard time believing she was simply born evil (though I imagine that sometimes happens), so most likely, her behavior stems from things that have happened to her in her own past -- which is just so terribly sad.

So the mother is a friend of a friend of your husband's?
I certainly wouldn't let her near my own kids or pets again, but I wonder if there's some way you could get help for that poor child?

she sounds like she is abused at home, mentally or sexually and physically or all of it. i don't know what to do about it, but sounds like behavior of abused child.

Yes that sounds very much like the child is/has been abused herself. Would it be over the top to suggest informing CPS of events?

To avoid future problems, I'd tell DH that his friend cannot invite company to any of your get-to-gethers. She apparently is not the best judge of character to bring someone into your home that could harm your child. Perhaps her elevator doesn't go all the way to the top but she certainly wouldn't even be invited in the first place. Family comes first and those invited into your home are guests you consider safe. Her judgement is not safe. Lynn

when DD was little we had two neighbourhood kids who terrorised the whole area, they were maybe 4 years old the most.

One used to take big branch, come behind people and smack them with that branch. Once he came behind me on a playground and hit me wiht the branch across my legs, my legs were covered with bruises. I was in so much pain and I was an adult, can you imagine he smacked children like that. he used to hit other children on their heads, can you believe it? his mom would just say: oh don't do that baby.

And the other little girl used to scratch and bite everyone pretty bad. I can remember few more kids who were that violent at young age.

apparently it is not uncommon, but I suppose they learn it from somewhere or someone? family or maybe violent TV?

It definitely sounds very disturbing, and abnormal.

Does your Dh know the father at all? Could anyone speak to him, since you say the mom won't address the problem?

I def. would not be comfortable having that child around your kids anymore. Scary.

We do not know the father at all. Here is some facts i know.
She got pregnant after 6 months of dating him. She is a veyr pretty girl but my Dh's friend and i were wondering if she did it on purpose to make sure the guy stays....
they married, live together for another 4 years after the child was born but out of the blue...and i do mean out of the blue, he todl her straight out that he doesn't want to live his life with her and siad he's going for a divorce. We first thought..ok...family man...doesn't want it..never aksed for it in the first place..but then we thought..mmmm.. affair??? nope..no affair just wants completely out, will pay her Cs with minimum contact, no communication whatsoever.
then i meet the mother...who is very intelligent. highly educated...but i noticed she wants to party and drink and mingle.
so i'm thinking,,,,,neglect. Maybe this kid just doesnt' get enough attention , get ignored and maybe she was born with some sort of psychological problems. Its rare..but it does happen. I immediately picked it up the moment she stepped into my household but didn't think she would be violent. And she realized soon i was completely onto her and she didn't like it at all. So she is very aware at her age what she is doing and she purposely takes the individual or animal, isolates it before she begins her assaults...so she KNOWS. She's 5 but she knows...I think that is evil incarnated! I've read other stories as well with kids rare, but are born this way. They have to be hospitalized immediately.
I read one on another forum where the daugther was given up by the mother because she was harmful to her new baby brother...then with the father and sm...she held a pillow over her baby sister! Well they all had her committed. Turned out she was a psychopath. No emotions, no empathy, cruel, maticulus and planned.
EIther way, i've told my husband i never want that girl over and told his friend that she as a mother should adress these odd behaviours before someone or thing get seriously hurt.

That is horrible and I don't blame you for not wanting to have anything to do with the child, but as a human being and a member of the community, I think you need to ensure cps is informed so the child can be treated and hopefully persuaded away from this path, she could indeed be a psychopath and you don't want to hear about it on the six o'clock news down the track.