Grant believed in states rights and in secession. I guess fighting a war that you think is fought for the wrong reasons would make one a drinker.
Grant had had an alcohol problem from the his time at Vancouver Barracks in the Oregon Territory and Fort Humbolt in California. He greatly missed his wife and son and dove into the bottle for solace. He resigned his commission soon after. There is very little documented evidence that Grant drank during the Civil War. In his Memoirs, Grant lays the blame for the Civil War on the Slave owning aristocracy of the South.
While he did drink, reports of over imbibing were exaggerated by jealous rivals. Early in his career he was in charge of temperance with the troops. Reading his biography right now.