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.
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.