In the sentence
What is the purpose of ものだ? It seems to me that ...刺したから... (because he stabbed) would work on its own. I could also imagine that ...刺したのだから... might work where the のだ is the explanatory-の thing. But I can't make sense out of the whole ...刺したものだから... construction.
PS. I didn't really stab the ogre but I couldn't find the right kanji for the correct name.