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.
Grant believed the south had a right to secede. I think he is fairly clear here:
The fact is the constitution did not apply to any such contingency as the one existing from 1861 to 1865. Its framers never dreamed of such a contingency occurring. If they had foreseen it, the probabilities are they would have sanctioned the right of a State or States to withdraw rather than that there should be war between brothers.