In chapter 3, why does the dealer reason that cowards shoot before it is necessary?

To understand what the card-dealer says in chapter 3, we'll have to go back to chapter 2.
In chapter 2, the unnamed narrator is watching a card game at a saloon. The Virginian (who is the narrator's escort) is playing. During the game, the Virginian (who is also unnamed) is addressed by Trampas, another player. Trampas is referred to as a "cow-puncher, bronco-buster, [and] tin-horn." The card-dealer tells the narrator that Trampas doesn't enjoy losing to a "stranger" (the Virginian).
When it's time for the Virginian to call his bet, he initially remains silent. This prompts Trampas to goad him with "Your bet, you son-of-a------." Now, the epithet leads the Virginian to lay his pistol on the table with a quiet invitation: "When you call me that, smile." Suddenly, the atmosphere becomes tense, and there is a general feeling that violence will break out.
For his part, Trampas decides to back down. However, the narrator notes that the challenger isn't happy about doing so, noting that a "public back-down is an unfinished thing."
Now, we get to chapter 3, where the card-dealer tries to explain the difference between the Virginian and a man like Trampas. The dealer tells one of the card-players that the Virginian was never going to shoot. For his part, the player demands to know how the dealer defines a dangerous man.
The dealer answers that the Virginian is a brave man and that this type of man does not constitute a danger to anyone. Rather, it is the coward who is dangerous. Such a man shoots before there is any need to. The reason he does so is because he does not operate by an honorable code of conduct. This is the main difference between the Virginian and Trampas.
The dealer explains why the Virginian was never going to shoot: the latter would only pull the trigger in the face of a clear threat. Since Trampas merely threw down an insult, the Virginian held his peace. The dealer maintains that it is the coward one needs to fear. This type shoots before it is necessary, as he is primarily motivated by his wounded pride. The coward is almost always dishonest about his shortcomings and hides behind a mask of false bravado. Thus, the coward acts without honor and, therefore, can never be trusted.
To support his rationale, the dealer tells the story of a trouble-maker who entered the saloon the previous Tuesday.
The trouble-maker apparently got into some misunderstanding about the drinks he ordered. Before the disagreement was settled, he shot two men, both innocent bystanders. The dealer tells the narrator that the trouble-maker was eventually "put out of business" and that he died that night. At this point, the dealer maintains that cowards are more to be feared than someone like the "black-headed" Virginian.
The dealer also explains that, by the time one worries about what the Virginian will do, it will be too late. When faced with a credible threat, the Virginian always acts swiftly. The Virginian is deliberate and also lives by an unspoken code of honor. This is what distinguishes him from men like Trampas and the aforementioned troublemaker.