How should I handle my fiance's violent, undisciplined ADHD 10 yr old boy...brat?

I've been with my fiance' for a year. Her 10 year old son is diagnosed ADHD and goes out of his way to have the worst behavior you can imagine! From early on in his life, he has had councelors, been in special schools with rubber rooms etc. When he doesn't get his way, he gets whiney and if that doesn't work, he gets violent and has trashed her house breaking down doors, smashing furniture, breaking windows, beating his younger sister, pissing all over the bathroom and on peoples's toothbrushes, threatening to burn the house down and kill everyone and the list goes on! He says "no" when asked to do anything and often simply refuses to get up for school and has missed over 20 days this year. He is on meds and refuses to take them. His mom is a small woman and is overwhelmed and overrun by him. At 10, he outweighs her at 150 lbs and I believe she is afraid of him. To me it seems that any kid who is a brat now is diagnosed ADHD. He is terrified of me and we have had some run ins, once after he chased his sister and my daughter with a knife! I'd like to develop a relationship with him but his behavior is something I can't ignore like his mother often does. His mom makes excuses for anything he does and is more concerned with why he does these things. I tell her it won't matter why when he's standing over his dead sister! He would like to see his mother dump me and does everything he can to interfere with our developing relationship. If I didn't love her so dearly, I wouldn't begin to deal with this! I must be mental myself! I'd like to take him out in the woods where nobody can hear him scream! This kid is truly dangerous yet I don't want to **** with "mother bear" either. She has asked me to please not tell her that I hate her son. I won't. This kid just now learned to read due to his non involvement in school but is calculating and intelligent. Soon he will be a 250lb 15 year old and in my opinion is a future convict. Aside from beating his *** or leaving his mom, what should I do? I usually come out looking like the asshole for suggesting he be disciplined in some way. He basically holds the house hostage with his violent outbursts! Here's the kicker...he's been this way all his life, but one year ago, his older brother who discovered he was gay, hanged himself in the family bathroom and guess who found him there! Now mom is afraid he'll do the same thing if she traumatizes him. She is a great mom and in no way caused her sons death. He was the type to bottle up his problems..nobody had a clue he was even depressed. I can't wait to hear the comments on this one!