There is an overwhelming body of evidence that once you say you are going to leave, your spouse's problems with alcohol or drug abuse will become even worse. Sometimes, they become temporarily better, but, without therapy or other interventions, they usually become worse fairly soon. You were a lousy spouse. The public setting will encourage your spouse to respond in a restrained and rational fashion. Break the news during the daytime.
Morning is best.
Break the news when you are both sober and drug-free, if possible. Be physically as much at eye-level as possible. Be prepared to drain yourself of anger before you begin, and during the conversation if you feel it welling up. You can feel sorrow, yes. During the conversation, take deep breaths to relax.