There is a difference between saying "X is not to be trusted" which regards X's character in general[Stating one's own conviction is not an attack on another person's character.]
Sarkus: It certainly can be. If it is my firmly held belief that X is not to be trusted, that is certainly an attack on X’s character, is it not?
and "I conclude from his arguments that X is not participating honestly in this debate" which is situational and demonstrable.
It presents a cause-effect relationship between present situation and his conclusion.How so?
It's a weak argument, and somewhat off-topic, but I still think it qualifies.
More like: "Your reason for believing that is faulty: these were my good-faith arguments."What would a valid counterargument look like? "No, you don't believe that."