It is well known that this man abused drugs and alcohol, so these things could have easily played a role in his mental deterioration. It is proven that the use of drugs and alcohol can cause damage to a persons brain, as well as many other health problems in the body.
Sure, it could have played a role in his mental deterioration. Most of the people who end up in prison usually have some sort of drug addiction, the most common of those drugs being alcohol. But I personally believe that there is real evil in the world, and that it manifested itself in him.
Yes, substance abuse, such as alcoholism, could have played a role in Jeffrey Dahmer's mental deterioration, because extreme substance abuse destroys the mind. Scientists have studied the effects of substance abuse, and have found that those who abuse alcohol literally lose brain function. This could cause significant mental defects and contribute to Dahmer's mental state.
Jeffrey Dahmer was undoubtedly an individual plagued by many mental problems, but his abuse of alcohol and other drugs certainly didn't help matters. For individuals with mental health issues, self-medication is common, but it often makes the issues worse. It's easy to imagine that Dahmer's alcoholism further aggravated his mental deterioration.
Even if Jeffrey Dahmer abused various substances, it does not explain his mental deterioration. The fact of the matter is that Dahmer was an absolute psychopath and would have been no matter what. Substance abuse does not make a person commit such heinous acts. However, a predisposition to muder does.