Thank you for your post.
I am so sorry about what you are going thru. It does sound like your foster filly does need to be euthanized for the safety of everyone, including herself. Have done rescue for 14 years, I am well aware that there are many irresponsible rescues out there or rescues who can't face taking a life
My apologizes for coming across as "bossy", but I recommend sending a certified letter stating that you have tried to contact them multiple times regarding this filly. Explain that because she is a danger to herself and others, she needs to be euthanized. I would state that if she has not been removed from the property by a certain date, she will be euthanized. I would also call and leave a message saying the same thing. That way, they cannot come after you for euthanizing "their" horse (who has been essentially "abandoned" at your place).
Bless your heart. It is a horrible decision to make, but given her behavior, it doesn't sound like there is another other reasonable alternative.