Interesting. My recollection was that as the books progressed, the plausibility of LV not caring about pure-bloodedness grew larger, but he himself didn't admit to any such thing onscreen. Then in Deathly Hallows he suddenly did care about it again, because Rowling suddenly realised that he didn't actually have a motive at all. So any old ill-fitting one would have to do, as part of the overall trainwreck.

Then again, my perspective on HP has been thoroughly polluted by the brilliant essays over at http://www.redhen-publications.com/.