I can see why your older son is scared and refuses to be alone in a room with his brother. No one is protecting him or standing up for him...He has to be an adult at seven. More than an adult really...adults can call the cops if someone is beating them up...and can file a restraining order if it's happening on a regular basis.
<grinning> Now here's my caveat...Your younger son sounds exactly like my son :) (and the son of a close friend of mine). Strong willed, intelligent, super active, funny, charismatic...born leaders or tyrants, etc.
My *experience* with these types of kids is that the teaching empathy stuff that works with more sensitive souls, doesn't work here. That a parent has to tie a pavlovian response of feeling bad, when they hurt someone, and that THEN the empathy stuff works.
What we did was to have some unilateral rules. Hitting was an instant scooped up and in time out in the crib. (We kept our crib for longer than most, because it was such a convenient timeout place). With the "You don't HURT people" and walk away with the door open...so he could very easily see us going on with our normal lives. If he was upset, good, we could work with that. If not, then we guilted him into feeling bad. Full on, full out, guilt & shame. I used to be embarrassed by that, but not anymore. So too, "if you throw a fit you don't get what you want". If you want to leave, we stay. If you want to stay we leave. ... Ahem... these things tie into the whole not falling for/feeding the manipulation thing. Nope. You don't learn to hit because you want to be by yourself for awhile. You learn to say you want to be by yourself.
At first, my son obviously felt badly for himself. Then you start doing the conditioned empathy thing. "You HURT your friend. How you feel right now, is how HE feels, but worse. You HIT him and that hurt. And someone who he LOVES hit him, which makes his heart hurt. As much as you hurt and are unhappy right now, he feels WORSE. You HURT him."
The whole point of course, is to bring on more tears and crying, to make them feel as bad as possible about what they did. And THEN to start giving them options; how to apologize, how not to hit in the first place (taking a deep breath and counting to five, saying you need alone time, etc.).
Once they start feeling badly about hurting others you can start tying in media. Pointing out how a charactor is losing their temper, and needs to start doing what THEY (the hitting/biting/pushing/whatever mean-violent behavior the toddler falls into when THEY lose their temper) is learning how to do.
The inverse of this whole thing of course, is that the flipside of these traits are VERY VERY good. So we praised and rewarded the heck out of them. It wasn't the toddler that was bad, we weren't trying to rewire HIM just his behavior in this one small area. The negative aspects of this personality lost him things, the positive ones gained him things. We wanted him to KEEP his personality, but hurting people isn't an aspect of personality, it's a learned behavior. As is NOT hurting people. Everyone flails at first. Well, nearly everyone. We regarded it as our duty as parents to direct that, to teach him how to USE his personality, and not to flail and lash out. (Wow...doesn't that sound pompous? <Laughing> in all reality it was a learning experience for all of us, with good and bad days...but I was very serious about the duty aspect. Even if it meant that I was late, or missed out on something, this was a thing that had to be gotten to and dealt with accordingly each and every time. Sigh. Soooo not easy. I don't mean to make it sound like it was.)
My friends tried timeouts with their son, but wouldn't guilt him into feeling bad, so they gave up... because he was just as happy to be on timeout as anywhere else (and like us, they don't believe in physical punishment). He's 5 now. While most of the physical injuries he's caused to others are minor, he sent an 8 year old to the hospital a few months ago because he pushed him down the stairs at a birthday party. His reasoning was that the other kid was older and was "in his way". He honestly doesn't give darn who he hurts, or how much he hurts them. At having to leave the party, he was upset about not having cake, but not about hurting this other kid. My friends didn't do any tieing in (even with this extreme situation), because they don't want him to "doubt his decision" or to "lose his self confidence". Um....HELLOO...
He finally got punched in the face by another little boy last summer (about time, he'd been torturing this poor kid). My friend's son bawled, but since the other boy went home (duh), my friends son in telling the story from HIS view on it is that this boy had to leave because he hurt him. He honestly thinks that it's okay for him to hurt others, but not the other way around. He doesn't make any of the connections. The whole situation is really really sad. Because (except in this area, where he's been given no guidance whatsoever...aside from those self same games that just don't WORK unless they feel badly about something to begin with)...he's a really sweet, bright kid.