Why did he cheat on me with my best friend and treat me like crap after? I did nothing to him!

We dated 3 years, had a very healthy sex life that was enjoyable for both of us.. at least I thought.. until I found out he cheated on me (1 night stand) with my best friend. My question does not regard my best friend.. it regards him. Looking back he never showed a single sign he was unhappy. He constantly told me how much he loved me, wanted to marry me, how hot and sexy I was, etc. If he was unhappy, he convinced me the complete opposite! He was a binge drinker.. but that is still no excuse.. or is it? Idk... I am a christian girl and he knew that I really value faithfulness and that in cheating on me period it was a deal breaker (esp with my best friend). Looking back, I realize that right after he cheated he felt bad because he acted weirdly nice, like the perfect boyfriend, planning to take me on all these special dates, talking about our futures etc. I had mentioned to him one day (turns out it was the day after he cheated) that my best friend had called me at 7 am and how I thought that was weird. Then a few days later we got into a small fight on the phone and after that it was like a switch went off inside him. He started treating me very coldly and meanly. was just really heartless. When the truth finally came out of him he made a comment "you should get better friends next time" as I cried and shook on the phone. What was with this whole tough guy attitude? I mean, he already did the worst thing he could've done to me... why treat me like crap on top of it? Is that who he really was? Or was it 4 his pride or to make himself feel better about himself after doing smoething so terrible? He knew he had made a mistake and that once I found out it'd be over, but he totally put all of the blame on her... and all of it on me by saying I "picked" the wrong friends. Did he start treating me like crap because he wanted me to feel like I'm to blame, so he didn't feel as badly about what he did? Or had he decided I wasn't worth it? (still doesn't explain y he treated me like crap..). Did he think my friend was better than me? He threw away my trust, love, and respect for a piece of her ass while he was drunk... so he must've! After all the time we spent together... I don't know this all came out of left field, I'm still confused about it and so is my family. Had he liked my friend all along and I just never knew it? Was he angry at me deep down and doing it as a way to punish me or is he just a horny bastard and the alcohol brought it out finally to the open? Do you think he enjoyed it/ thought it was worth it? Or do you think deep down he regrets it, but is just too prideful to admit it to himself or anyone else, because he knows I'll never take him back? Idfk. I just had no idea he looked at her like that, especially when he had me. I feel like I was used and completely fooled for 3 years... I really loved him, but I wasn't enough for him. The worst part was how he treated me after... he couldn't just say sry and leave me in peace, no he had to treat me like sh*t.
Why did he cheat on me with my best friend and treat me like crap after? I did nothing to him!
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