On the chemical level, the toxicity of ethanol is mainly mediated by its breakdown product acetaldehyde. Alcohol is metabolized like this:
ethanol → acetaldehyde → acetate → acetylCoA → CO2 + water
When alcohol is drunk in small amounts, acetaldehyde is quickly metabolized to CO2 and water, but when drunk in large amounts, the greater amounts of acetaldehyde can damage the cells, mainly in the liver, pancreas, brain and heart.
Alcohol breakdown also results in the creation of reactive oxygen species, such as hydrogen peroxide, which together with acetaldehyde attack certain parts of the cells. The damaged cells attract neutrophils, which, in the attempt to clean the mess, can cause further damage (more details in the Journal of Hepatology and in this nice video).
Alcohol breakdown also results in an increase of NADH and depletion of NAD, which stimulates fatty acid synthesis.
On the tissue level, the stages of alcoholic liver damage are fat accumulation (steatosis), inflammation (hepatitis), cell death (necrosis) and conversion to fibrous tissue (fibrosis and cirrhosis). Chronic alcohol consumption also increases the risk for cancer of the mouth, throat, esophageus, colon, liver and breast (CDC. gov).
On the functional level, alcohol can kill you due to:
- Brain impairment:
- Bleeding in the brain
- Depression of the cardiorespiratory centers in the brainstem
- Wernicke-Korsakoff syndrome due to alcohol-related vitamin B1 deficiency
- Hepatic encephalopathy caused by excessive amount of ammonia in the blood in advanced liver cirrhosis
- Heart failure:
How exactly alcohol is damaging? (See Table 1 in this article)
- triggers triglyceride accumulation in the liver cells, which can result in cirrhosis
- causes DNA damage of the stem cells, which can result in cancer
- triggers inflammation, which can contribute to liver fibrosis
- damages DNA
- promotes oxidation of LDL cholesterol, which increases the risk of atherosclerosis
- inactivates clotting factors, which increases the risk of bleeding
In conclusion, alcohol does not kill you by causing a chemical reaction with a certain molecule in the body (for example, like cyanide intoxication) but by promoting or inhibiting certain processes that can result in tissue damage.