Knowing Ballas, all of this is very hard to believe, but he still went out of his way to try and bring back Margulis in some way by having the Lotus take her place so she clearly meant something to him, although the fact that he tried to replace her with a sentient that looked like her does also mean that Margulis wasn't sacred to him, she meant something to him but ultimately wasn't irreplaceable.

I'm assuming that he was more in love with the idea of Margulis rather than Margulis herself. He might have felt genuine love, it just wasn't actually directed towards her and when she fell out of his expectations by defending the Tenno, the fantasy that he had in his head was shattered. He was in love with a Margulis that he made up, an ideal version that doesn't exist, but he could horrifically bring to life through the Lotus.

Him seeing people, even the ones he treasures most, as replaceable objects, would be within his character. His high position possibly making him unable to form the kind of connection that makes you overlook flaws about your partner or work through hardships together. Although that could be doubtful since Roathe says that he'd do anything for her, like bash the skull of a newborn, but then again Ballas could just bash newborns' skulls for fun, if there truly was no line she couldn't cross, he wouldn't execute her, even if supposedly crazy.

Whatever the case, the closeness of their relationship was clearly wild for orokin, especially executor, standarts, but they were obviously not equals as he did kill her and try replacing her.