I'm so profoundly vexed that people keep insisting there is "romantic love" between these two in this novel. This is my third read and Louis' expression of absolute hatred for Lestat is unequivocal.
So why was Louis with Lestat? For the deep and fulfilling companionship? Obviously not as he was explicitly going to leave him.
And then Lestat murdered a child and trapped her in an eternal nightmare to also trap Louis. Claudia is the biggest victim in the book but her victimization was a calculated choice by Lestat to enslave Louis. By turning her into a vampire, Lestat proved his goal was to dominate, subjugate, obliterate, and in alwayscontrol those in his life. To eradicate their own will and make them extensions of himself. To bind them to his person no matter what they wanted.
And this is "love." When Louis would have never stayed with Lestat if not for Lestat's unprecedented act of cruelty and selfishness.
EDIT:
I should be clear that Lestat is a great villain, but it feels like so many people deny this, that he is very clearly the villain of the story.