I might have been looking at r/HPMOR when I saw it. To be fair, preserving 37 heads would have taken a long time- as in, he might not have had the magic to cool them all, so most would be warm and dead anyway. It would also be hard to maintain the transfigureations.

Actually, I had just assumed that Harry had decided that Lucius deserved to die, and had blamed it on LV partially so that he could continue his friendship with Draco.