With the level of violence being such that the kids know to go to your room behind a locked door, you really need to take the step of reporting this to the police or whatever agency is there to protect children.
I am not speaking from just what I think. I had a violent teen. Before he got to age 14 we had to have him removed from our home. We were not dealing iwth his children, just his siblings. My younger children would go and lock themselves in my room when Wiz raged. (this was after many interventions, therapy, medications, everything we could think of/dream up to stop the violence). It came down to either keeping my violent child in my home and jeopardizing ALL of us, or having him removed and pressing domestic violence and assault charges against him.
It broke my heart, and nearly took my sanity, but I had hte sheriff remove him. I pressed charges, I went to court and testified as to what he did to us. We ended up iwth my parents choosing to take him in and keep him at their home. It made a big enough impression that he did start to make changes. Now, 3 years later he is stable, in school, working his first job, and actually has thanked me for forcing him to grow up.
You simply must put the welfare of the young children over the comfort/welfare of an older teen who is dangerous.
There really is no other choice. It may take hours of phone calls, of exposing the ugly behavior to the harsh light of scrutiny, and other equally horrible things to change the situation.
I guarantee that he will not starve - if he is 18 he will figure out how to fix food or get food he can eat with-o fixing. He will find ways to get help.
But you must take any and every step to protect the young children. It is what it is.