I'd ideally like to give this an 8.5 out of 10 (and I don't give 10s to anything). It's one of my favourite Maiden songs, partly because of the sheer energy, and partly because I've studied Cathar/Occitan society, the Albigensian Crusade and medieval history in general and I'm fascinated by it.
The ‘happy’ or lighthearted parts, I agree, end up detracting from the song, I'd rather they were shorter. The ending, in particular, seems over-cheerful, given the subject matter. However, I don't see it as being entirely out of context in this song.
It suggests a fanfare of triumph and glory to me. I've no idea if this is intentional, or just something I get out of it. The triumph in question could be either the 'victory' of the Crusaders in slaughtering the Cathars, supposedly in the name of God, or the martyrdom of the Cathars, staying true to their religion but taking all their followers with them to a horrific death. Taken in this sense, the contrast of the 'happy' sections further emphasise, at least to me, just how misguided and out of place these causes - ultimately done in the name of something moral and spiritual - were in light of the terrible human cost.