it's hard and feels really crummy to be the "bad guy". but you aren't the bad guy; you are doing nothing wrong to want peace and order in your home. and to protect your younger son.
continue to set those boundaries, stick to them, and when he pushes - which he will - re-establish them and try your best to stick to them. it is a learning curve for you BOTH. you'll both get it...and then he'll stop pushing when he knows the door (figurative) will stay closed.