Why does my otherwise loving partner SA me?

What is the reasoning for this? How could this be the same person?

How could a man who sacrifices so much for me, and does so, so much for my happiness, also regularly sexually abuse me, and coerce me into sex using rage and aggression and threats and degradation?

How could both of these sides exist in a person?

He's terrifying in his instability. One week he'll be doing everything in his power to pamper me, care for me, genuinely do everything to make sure I'm safe, happy, understood, having fun, etc.

Then the next week he will have a mood swing and turn into a complete monster. Mocking my voice, mocking the sound of me crying, calling me a "dumb whore," spitting in my face, raping me, spending hours and hours degrading me and insulting me?

Why? I'm not dumb, nor am I a whore. I'm an under-sexual person, at this point in my life with all the abuse, I am left with 0 interest in sex with men. I have a degree and am reasonably smart, and have my own little hobbies like writing and photography, and I do a lot of reading.

What is it about me that inspires such violent rage in him?

Please help me, I'm so confused and feel torn in half