A More Permanent Hell

Plus, Methos does lead Duncan and Cassandra to the base, and he kills Silas rather than Cassandra. Methos's pain is also so obvious at the end that I don't think Duncan could let her do it. One of the things Duncan is confronted with throughout the series is to see the world in shades of grey, and I'm guessing he *thought* about Methos's hundreds of years out of the Game, his (though rather ambivalent) help in tracking the other Horsemen and the man Duncan had come to know - and yes, made a judgement call not to kill him or let Cassandra do it.