I think it’s questionable whether Kinzo intended to cause the massacre. Remember, we have a precedent for this sort of guilt assignment in that Battler was responsible for the 1984 massacre because he came back and the massacre would not have occurred if he either stayed away or kept coming to family conferences even during his fight with his dad. Similarly, the first massacre might not have occurred if, for example, Kinzo wasn’t there to be a translator and thus accelerate the break-down in negotiations that lead to the slaughter.
Also, look at the exact text here. Kinzo is being called a coward for even saying anything about stealing the Itallians’ gold. But of all the many things you could say about a thief, cowardly isn’t the appropriate insult here if he was proposing murder and thievery out of the blue.
However, if it was something like he refused an order and accused the thug-commander of wanting to do that, thus putting it in his head, and his fellow soldiers were scandalized at the idea because they didn’t yet realize what a vile piece of work that guy was, things seem to line up with that scenario.