If a family member has been diagnosed with scitzo-effective disorder how should other family members cope?

He behaves as if he knows everything and everyone else is wrong. He fights about everything- threatens people, etc. but he also is very depressed 99% of the time and it's hard to not hurt him and add to his depression, but at the same time he is angry all of the time especially when other people dont agree with him. while its very hard not to hurt his feelings, we are afraid that he may one day act out his threats( which re seriously frightening- murder and stiff like that- he says like 'I'll slit your throat and laugh as you bleed) it's disguisting and very scary. He refuses to use medications and go to therapy. He thinks he is fine and he doesn't need medications or whatnot. Does this disorder get worse or better without treatment? The family is having a hard time coping. He has become like a completely different person. The obvious answer is to have him admitted in a psychiatric hospital- against his will. But we fear he will never come out and if he does he will be so angry that we forced him to do that, that we will all be in danger. also there is a fear that he will use pills to overdose when he is in his depressed mode. Has anyone ever had to cope with a family member who is suffering from this disorder?Need advice before it's too late! (this is my son, who lives with my elderly mother and he is 20 years old and has had this problem since a very young age- 12.)