Nostalgia for the good times, that’s all.
I think that’s normal, regardless of what an arsehole he was the rest of the time.
And, TBH, I think it would be the same even if he wasn’t an arsehole and you just split because you weren’t the right people for each other.
That chapter of your life is now closed and you’ve moved on but it’s okay to remember the good times fondly from time to time. Just give yourself a reality check every now and again and remind yourself of his Jekyll and Hyde character and the reason(s) you split.
I occasionally think of my exes if something reminds me of them – a song, a smell, a place, etc. I have a few minutes of wistful daydreaming then get on with my day. Most of them were decent guys, barring one, and I know I don’t want to get back with any of them.
Just memories, that’s all. Part of life's rich tapestry.