It's because of selfishness, Wolfy. I believe that the root of all evil is not, as the Bible would have it, the love of all money but rather selfishness. Selfishness leads to pride, to greed and to a lack of empathy and therefore comapassion for others. All of these things together vring about those actions that I would consider to be evil.
So in the case you're thinking about; selfishness led the people you're talking about to want to keep their memories of their relative pure and so to achieve this, they denied his lover his memories. Not to mention that greed gave them a powerful motive for denying him everything else too.