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I reccomend you get yourself a good chicken book and hang around here often. There's alot you need to know.
And the mind and behaviors of a chicken can be complex at times.
Firstly, your new hen attacking newcommers is 100% normal. She's protecting her own territory. Assuming she's been in your home for a few weeks now, this is her turf. That means she'll feel the pull on it and show the newbies that they cannot invade her home without going un-noticed. But in time, she will accecpt any newbie. They just have to go through a sort of initiation. It is sad that your two friends ran away (or were eaten), but just because you're raising your own house-chicken, it doesn't mean the others will take to it well, especially birds that you have not hand-rasied from chicks. You can't just let them go and expect them to stay/have your hen babysit. They don't know what home is. The house-pet behavior your hen shows is a bit of a rarity in the chicken-world. Although she is pampered indoors, if she was to begin a new life outside, I'm sure she'd adjust and it would come more naturally.
A few questions for you:
-Have you established the age of your hen?
-How old were the birds you had bought that ran away?
-What are you feeding her, exactly?
She WILL absolutely need buddies. But they should be roughly the same age as her, if not, a little bit older. She'd dominate young birds to the point of possibly killing them. Also, if you get any new birds, they should be quarantined for at least a month. They might have outside germs or your hen might give them something, and they should not mix until you make sure BOTH groups are healthy, that is, in that critical month.
For her own safety, I suggest a small coop enough for at least one buddy, probably two to be safe. Sleeping in a tree is okay now, but as it was previously stated, what happens in the winter? Also, some predators will be able to get her once they figure out there's something tasty hanging in a tree at night, and chickens really can't protect themselves.