Hi Alex and welcome to SR,
Unfortunately, alcoholism is a family disease as Pelican said and effects the closest family members to the alcoholic the most. Those that have to live through the daily ups and downs, walking on eggshells and the uncertainty of the family unit staying together.
I am sure that you have left an awful lot out of your post and have no doubts that you are having to deal with far more than you mention.
Arguing in front of your children is never right or healthy and in itself is a form of abuse but it sounds to me that your parents relationship has reached the unhealthy stage, in the depths of alcoholism when irrational, unbalanced behavior patterns take over and neither know right from wrong anymore.
I personally think you sound like your dealing with the situation fairly well, as your showing signs of spunk, know what your parents are doing to you and your siblings is not right and more importantly are questioning, seeking knowledge and support through this website. IMO all good signs. The homicidal thoughts come from being angry with the situation and still being emotionally immature enough to deal with it and again IMO nothing to worry about. It would benefit you greatly though if you could discuss your home life with your doctor who may be able to help with some counseling/therapy sessions to help you with some strategies to help you cope with stress and its triggers.
My own daughter who is 19 yrs has had to cope with an abusive alcoholic father for a couple of years now (to her directly) and is moving out of the family house this weekend and house sharing with a couple of her close friends. She is relieved to be leaving and is looking forward to some peace from the situation. I am pleased that she will be out of this toxic environment and getting herself healthier. She is well educated in alcoholism, she didn't take to Al-anon (it wasn't for her) but has seen a therapist which I know helped her a lot. She knows that her dad has a disease and trys not to take what he says to her personally, which I know is hard to do, especially at an age when she is maturing.
I too, am a mum who has tried to argue, manipulate, shout, swear at, question, discuss, confront, plea, threat and issue ultimatums such as separating or the 'D' word for 22 years but nothing has worked - and I have made myself mad in the process. I will admit that there has been times when I have been secretly pleased that our daughter has witnessed these verbal onslaughts in the hope that my daughter witnessing the abuse may bring my AH to his senses - now that's messed up thinking right there, that I am not proud of. My AH has progressed even further now and he doesnt appear to have a problem with abusing both of us at the same time!
At 20 yrs old, I would say that you have a right to remove yourself from the situation (if you can). Try Al-anon/Al-ateen, seek out some councelling (via your doctor/school/college) Read up about boundaries and set some for yourself. e.g. Pick the right time and ask them both not to argue in front of you and your sisters and if they do, retreat to your room and listen to music with headphones in or take your sisters out for a walk.
Good luck Alex, I am thinking of you.