Torch has probably the largest soft spot of any dragon I've seen that hasn't interacted with ponies. Namely, that he actively watches out for his daughter and prevented her from competing for the staff because he felt she was too small and weak to make it through, thus not wanting her to be harmed.If Torch was aware of the treachery of this dragon lord (assuming the story actually happened at some point and considering dragons like to brag), it's likely he took over in a relentless fashion but brought a level of decency to the dragons for the time he ruled. Hence the dragon migration not razing ponyville.