Merriman is a really really old guy who has seen and lived through many wars and battles. While this leaves a mark, so to speak, he doesn't usually talk about it to just anyone. Usually dark thoughts crop up out of nowhere as something reminds him of it and he makes random odd comments.
Examples of what might come up in conversation or introspection: physical and psychological warfare, careless treatment to others like him (he's gone as far as making Will very ill so Will would be in the right place for a quest).