Yeah, I pretty much agree on blackmailing. Although I had a thought that Duncan just saved a guy, then, while attending to his wounds, saw that the guy in question was wearing Luskan armor or something... but by that time it was too late. After all, it's damn hard to kill off the very person you just saved from death (some people do get soft towards those who are helpless and depend on them... or, more precisely, to their own 'charity', makes them feel good as 'helpers'
) - so the whole blackmailing stuff could be used not only for his own benefit, but also to keep a rather dangerous individual in check ('Yeah, I saved you, now you'll be a good doggie and won't burn any other villages, right?')