I hate my Stepson!!!?

First off let me just say this, if you are going to have a biased opinion in regards to what I'm about to say, read no further. I am engaged to my Fiance for 3 years now. We have been together for 4 years. He has a 10 year old Stepson who has been living with us 5 days a week for going on 2 years now I believe. My Stepson's Mother at the time had given up on him and told my Fiance "If you don't take him I'm putting him in a group home!" Needless to say, she had had it up to there with him! He was ornery. It was tough at first when he started living with us. He wasn't always apart of his Father's life and I blame that on my Fiance but more specifically my Stepson's Mother. At first things were really bad. I mean, BAD! His kid was adjusting to a new home and a new life with us and he was not liking it one bit. He hated me and I don't use that word lightly. He HATED me! I can honestly say the feeling was mutual. He was a total brat! As time went by, he came to liking me as I did him. I even grew an honest love and affection for him. I treated him as if he were my own. Up until about 3 months ago I'd say, things have taken a downfall. He argues, disrespects, lies, talks back...just a total attention seeker in the most negative way. I call him a little narcissist. He never takes accountability for anything he does and everything is always someone or something else's fault! I'm so sick of it. His Father works about 50 hours a week and never spends any time with him and his Mom only see's him on weekends. I feel like I AM THE PARENT. Not even a biological one at that! It's extremely frustrating. So much screaming and yelling goes on in my home and I am tired of it. I am so stressed out and I know I am not the only one. I feel like the evil Stepmother who tells her Husband to send the kid away but honestly, I feel like booting him to China!!! The kid is so damn needy and co-dependent on everyone and everything. I'm surprised he doesn't ask me to wipe his butt for him after using the bathroom! He antagonizes my animals and shows force towards them when the growl or hiss at him. Just today he pretended to choke my 6 pound chihuahua and punch him in the face. I am very protective of my animals and immediately responded by screaming at him. I don't know what his problem is but I can't take it anymore! I've made it clear to my Fiance that further action needs to be taken. Maybe taking him to juvenile detention for a tour so he can see where his bad behavior will lead him in the years to come. Something. Anything! My Fiance takes no time to push further on the issue! It's sooo annoying! We had him diagnosed with ADHD and he takes medication for it. He's been seen by a psychiatrist but that did no good either. I feel guilty for having the feelings that I do. I feel like the evil Step-mom from Cinderella or something. All I want is for this kid to act normal. I wasn't brought up by having my parents scream and yell at me or curse and I never wanted that to be the way my household was run but I get so frustrated that screaming is my way of releasing my anger since I don't believe in using corporal punishment. I have never and will never lay a hand on any child. I feel like a trashy person every time I scream or yell but all my other approaches have not worked with him. I feel like it's the only way to really let him know I mean business. Please for the love of God, someone give me some sound advice loony I end up in a looney bin!!!