Why do alcoholics have terrible attitudes?

Not all do. If an alcoholic stops drinking and gets with the program (12 step program) they can work towards losing the terrible attitude.

Maya ---Their brain gets it's signals all mixed up.  The brain responds by dispersing those mixed up signals the way they are mixed together resulting in one ugly blend of anxiety and depression turned upsidedown and  sideways at the same time creating a jumbled mass of confusion.  Just like this message. 

If you understand what I just said you will understand why the alcoholic acts the way he/she does.  It's because his/her mind went bizzerk and crossed all it's wires trying to figure out what it is he/she really was trying to say and do.

Be happy! --- C1

Alcohol changes people when ingested. Character defects become a part of the person and stay with them even after (when alcohol is not in the system) if those character defects are not worked on...enter AA then they will persist. And not to mention the brain cells killed, and the possible liver damage. Along with the liver damage can come increased ammonia levels in the brain which can cause bizarre behavior.

It's because it becomes a need and the brain commands the alcoholic person time after time to drink alcohol because the body needs it. As it became chronic it will affect the brain sensation and some of the brain's functions. It is like an addiction that the person having this addiction will do anything to taste or to binge drink alcohol until all senses are numbed.

I hope this answer satisfies your question.