Depression is wicked in that it has to find a reason to feel bad. I wish that I could say that what you are feeling right now is unusual but unfortunately many people suffer just as you are now-and you will get through it. I think that the most important thing is to learn to understand what this type of depression is. It's a chemical imbalance and in your case most likely set off by hormone changes. I'm not a Dr. and I don't want to sound like I am but I have suffered from clinical depression so I understand how your thoughts and feelings feel like foot steps on shattered glass. The thing is, start to tell your brain to think differently. Even if you don't believe it, when you have an unkind or unpleasant thought say, "While I feel this, it is not the truth and then tell yourself the opposite. It will be exhausting at times, at times it may even feel impossible. I have two children and that was my string of sanity and my desire to live, so hang on to the truth that your brother not only needs you, he loves you. Don't accept the feelings you are having now as who you really are. And most important make a few calls and go see someone at your school who can help you through this. You are not alone. When you get past this-and you will, reach out to someone who is going through what you are and help them.
Well the fact that you feel empathy and sympathy for your brother shows that you're not a psychopath and you're not a monster. You are a depressed kid with anger issues. Talk to a counselor about it. You have the rest of your life to look forward to. You wont be stuck in the same house as your parents forever. Death and the death of others are not the only way out. You need better coping mechanisms.
Call a suicide hotline. I know, you aren't talking about killing yourself, but if you tell them what you just told us, they can point you in the right direction. They will give you a number for someone who can help you.