Haha, how coincidental. I just (and by just I mean less than 5 minutes ago) beat the Pokémon Kalos Elite 4 with a team that included my Sneasel named
Bellatrix.
Odd.
Well, anyway, I have next to no knowledge about
Harry Potter, so that association is moot for me. I only think of it as a star name, so I put it in the same group as other astronomical names like
Vega,
Lyra,
Adara, etc. I find
Bellatrix to be very pretty, and I agree that meeting a
Bella whose formal name is actually
Bellatrix would be quite refreshing (although I personally prefer the nickname
Trixie).
Bellatrix is feminine and strong, beautiful yet dangerous. I think it's definitely usable, except that it's apparently associated with a not-so-nice character, from what I glean from the other comments. I don't think I'd worry about that, though. Aren't the HP books/movies over now? I doubt it'll still be a big thing by the time your little one is old enough to care.