The Dark Lord did not love Bellatrix, she was simply a means to an end.
Voldemort may have wanted a child to preserve the Slytherin bloodline, as he was the last living descendant. We know that he was the descendant of Salazar Slytherin through the Gaunts. His grandfather was Marvolo Gaunt. We know that Voldemort had strong thoughts on blood purity, even if he was a halfblood himself.
Bellatrix Lestrange was his most loyal servant, and she was a pureblood as well. Bellatrix must have been thrilled to bear Voldemort's child.
“I was and am the Dark Lord’s most loyal servant. I learned the Dark
Arts from him, and I know spells of such power that you, pathetic
little boy, can never hope to compete”
Voldemort has demonstrated that he valued Bellatrix more than any other Death Eater. Interestingly, while escaping from the Ministry of Magic after his duel with Dumbledore, he took Bellatrix along with him, and then disapparated. It is notable that he did not do this for any of his other Death Eaters, such as Lucius Malfoy, who was caught and imprisoned.
“He was there!” shouted a scarlet-robed man with a ponytail, who was
pointing at a pile of golden rubble on the other side of the hall,
where Bellatrix had lain trapped only moments before. “I saw him, Mr.
Fudge, I swear it was You-Know-Who, he grabbed a woman and
Disapparated!”
However, it would be quite erroneous to say that Voldemort had love for Bellatrix, since she feared him too much for that to be the case. She is terrified, when she realizes that the prophecy had been smashed, fearing punishment from the Dark Lord.
“Your dear old mate Voldemort knows it’s gone! He’s not going to be
happy with you, is he?” “What? What do you mean?” she cried, and for
the first time there was fear in her voice. “No!” she screamed. “It
isn’t true, you’re lying! MASTER, I TRIED, I TRIED - DO NOT PUNISH ME”
Therefore, Bellatrix Lestrange was probably just a means to an end, as far as the Dark Lord was concerned.
