Severus is a big believer in looking for the soul of a person through their eyes; his world starts with the eyes of others. It's part of why he loves Lily, with the soft glow of green in her irises and the way they never judge him even if they never seem to see him, and a very small part of why he avoids Bellatrix. Bellatrix's eyes are too bright, too intense, like if he were to stare at them for too long his own eyes would burn and become blind. He comes to realize it's not because she sees the world clearer than everyone else, but because she has a hard time seeing it at all in the same relative dimensions as the rest of the world. It puzzles Severus that Bellatrix seems unaware that she's burning a haystack to find a needle - insane has seldom equaled stupid, in Severus's opinion, and Bellatrix isn't an exception - and it takes him a while longer to realize she doesn't even know she's looking at all, and then all Severus has left to wonder is why he watches her so, though she scorches and blinds. It's when Bellatrix finds her needle that Severus finds his own.
Bella's intensity doesn't wane when there is mention of Voldemort so much as it drifts and latches firmly to her subject. Bella finds her anchor, her needle in Voldemort, and it's hardly a wonder. They view the world in the same wicked, two-dimensional terms as one another until Voldemort's gravity crushes Bellatrix and pulls the wreckage of her into his orbit, and too soon all she ever speaks about with pleasure is him. It makes Severus pull away from Lily, because this is what hooked him in Bellatrix's eyes, this morbid fascination with her fires. Hers was the intensity of Severus's regard for Lily, and he won't let this realization go to waste. He won't lose himself for lost battles.