Heartbroken re decision made?

My 18 year old son has had mental health issues for many years namely due to horriffic sexual abuse he suffered at the hands of his bioligical father & grandfather. Since he chose to file charges with the police, his biological father committed suicide, his grandfather was called to trial but the prosecutors felt there was not enough 'evidence' to take it to trial & in the publics best interest to withdraw all charges from the grandfather so he got off scott free. I have had my son in multiple private hospitals seeking the best of psychiatric care for the last two years since he left school. He is addicted to drugs, pain medication, marijuana, you name it. He has attempted suicide a multitude of times & last night was the final straw in that he terrified my 3 year old son (his step brother), myself & his step father by indicating we were at harm & to lock ourselves in our bedroom throughout the night, that he cut himself up with a knife across his arms & legs & my fiance had to wrestle a large carving knife from him. We had police come to the house, he had a seizure in front of them & the ambulance arrived to take him to hospital but he walked out of there today as our mental health system in Western Australia is pathetic, they release mental health patients at the drop of a hat. We have told him many times previously that we cannot keep putting up with him living under our roof with his addictions that he doesn't seem to want to address & he is scaring his little 3 year old brother so he has to move out. I am heartbroken telling my own child he is not welcome in our home. He told me today we don't care about him, we don't love him, we only love his little brother, he is our only son & he is going to kill himself & we will have his blood on our hands. I have for years tried so many hospitals, psychs, taken so much time off work jeopardising my job trying to help him, I recently secured him a job & we thought things were going well but he took days off on drug induced illness. I can't help but feel guilty about our decision to have him look for crisis accommodation. I have tried to intervene & help him do this today but he told me to stay out of his life. I feel like I'm a terrible mother & have let my own flesh & blood down.