There are two other instances (in the series, not counting the Little Sisters of Eluria) with Roland displaying a similar mercy: In Lud, he doesn't shoot Tilly (one of the Grays) when she begs him not too and in the Calla he doesn't kill Slightman, mainly because Jake implores him not to.Maybe we should consider even more comparable instances to get perspective on this decision. Like, why did Roland let the breakers go? Again, I don't think that he's generally so very unforgiving.