I remember doing Sadism and obsessive love in A level physcology.
Basically, sadists have what is called an Anitsocial Personality Disorder in general. What is interesting about Bellatrix is that generally, people with this disorder have no respect for authority, yet she has a great amount of respect for Lord Voldy.
Antisocial behaviour disorders are in Cluster B of the DSM-IV Personality Disorders, which means basically, sufferers can be dramatic- like Bellatrix. But they are one-step down from schizophrenia and one step above OCDs. They like to dominate and delight in the suffering of others. Fits Bellatrix perfectly.
They can idolise someone, want closeness, but they cannot comprehend the feelings of others. They are obsessed with that person, and will convince themselves that they are in love with them, when really it is just a manufactured feeling. She may have believed she was in love with Voldy, but I personally think she was infatuated by him. He too, seems to have had a similar disorder, so maybe that's why it worked. Who knows?
It's all the way you percieve it. If you are writing from Bellatrix's POV, then it doesn't really matter whether or not it was true love, because it's what she believes.
Personally I think it was the fact that she couldn't dominate him, because you've got to remember that Sadists are very clever. I think that drew her to him, and she was in lust for a very long time, not love.
I bet JKR never realised she would have Physcology students analysing her characters!
Anyway, I hope this helps! x