It don't think Cadance directly "killed" Sombra. Sombra was destroyed by the energies of the Crystal Heart if I remember correctly, all Cadance did was rescue it from him.
In any case, what I do find to be ironic is that, as you said, Cadance is the most stereotypically girly character on the show and yet she manages to subvert the typical "Damsel in Distress" character that we usually expect from such characters. Not sure if this was intentional or not but I think it fits with the show's motif of "You can be girly and a strong person" rather well.