What is normal 12 year old behavior?

This sounds like a step-child?? If that is the case then yes, the child does have a reason to hate you in his mind. You are imposing upon his mother and this is painful for him and so he is acting out his pain with angry and violent behavior. This is quite common~ I would say in this case that you and his dad will need to have a very honest discussion with him, connect with his pain, apologize for any harm he feels you have done and express to him your desire to be his friend, not his mom. You'll have to work hard to gain his trust and respect....... Very hard~ I sure hope you don't send this child away, you are the invader not him if you think about this situation. It would absolutely devastate him for years to come and he hate you both for it and may turn out to be a violent adult~ Love, Patience, and understanding~

I am just the nanny. Mom walked out on them in October and has little or no contact with her kids. We have tried everything from being nice to being nasty. He has been in trouble with the law before I got here and since my arrival, his behavior has gotten steadily worse toward his brother. today, he brought his brother's desk down on him and thought that it was funny. I can't control this child and fear for my safety and the safety of the other two children in my charge.

You sound frustrated (with good reason) and definately may need more help that what we could tell you in any post.  There are some great books on discipline and teenage parenting which may help http://www.cjkidz.com/parentsgrandparentsteachers.html  Click on Teenagers Raising and Understanding. 

I do believe kids need love the most when they are acting unlovable.

Oh wow, then you really have no bearing on the situation at all. His father needs to remedy this immediately! I would call the police if he ever threatened me or put his hands on anybody while in your care, he needs to learn a good hard lesson! I hope your employer takes this seriously because you really don't have to put up with this, I would quit if this continues. You are right in thinking that you are at risk not to mention the other people involved. Please keep us posted~

Well, you do have to think of your own safety, but the father of the child needs to think of the child's wellbeing. No child should be that way, so he is obviously suffering greatly. I would imagine he feels abandoned by his mother and who knows what else is going on in his mind. He needs to be evaluated and helped, but you need to protect yourself and the little brother needs to be protected by the father.

I totally agree that kids who are the hardest to love are the ones that need love the most. Definitely need outside intervention, counseling etc...

Dad is also at his wits end, but he says one thing and does another. One night he says that he will take away the video games (Tour of Duty namely) and by the next day, hasn't done so. He told the 12 yr old he had threeoptions 1-behave; 2-go live with g&g in Nevada or 3- go to juvie. He hasn't followed through on these threats although I firmly believe that he needs to be removed from the home, school be damned. Today, I hear the 10 yr old screaming and go upstairs to find the 12 yr old hitting him with a belt! This is ridiculous. He behaves for al of a day or two tops and then reverts back to his assinine and mean and malicious ways!