Honorable, in Forrest's case, would clearly have to be defined by what was considered honorable in his time. For example, if someone called you a pup, you were perfectly justified in getting a gun and shooting the guy in the back - which is basically what Jefferson C Davis did with Bull Nelson. Nelson was murdered but Davis had a 'matter of honor'.
With Forrest, the fight in the plaza of Hernando was a duel not a brawl, and the perimeters were set out before the affair even began. Three men appeared to challenge his elderly uncle over the custody of some children belonging to both families. The men timed it to coincide with Forrest's absence but he was told of it and appeared - he was told it was not his concern. However, honor said it was his concern. He was the target's nephew, family, and one defended family. It was also a simpler matter - 3 young men against 1 old man needed some evening up! The offended party picked their weapons - shotguns, sticks and bowie knives - and they fired the first shot. Now, honor was served when the killer of the uncle was shot dead by Forrest, but justice was not - so the fight continued until it was resolved by all four being wounded, two dying.
The Forrest/Gould affair was another matter of honor. Gould was insulted by his demotion for losing his guns at the battle of Days Gap - he considered it a slur against his courage and therefore a matter of honor. He got Forrest into a hallway to talk to him and then used the opportunity to try to draw a pistol from the pocket of his duster - knowing Forrest did not have a pistol or any other obvious weapon with which to defend himself. Gould did it properly, issuing a challenge so that Forrest would know this attempted murder was a duel, by saying, "One of us will not leave here alive!" As it happened, Forrest was shot but gave as good as he got with a penknife he opened with his teeth. These two duels are examples of a certain societal code of honor we might not find so honorable today.
Brian Steel Wills examines Forrest's life from the viewpoint of the Southern code of honor - A Battle From the Start. It's a very interesting book and may help explain some things about Forrest that do not seem to fill the bill for honor.