don't believe an exW would become bitter and crazy just like that
I would have said the same around 8 years ago...
dps ex was lovely, they had a great amicable co parenting relationship, did eachother favours, were sorting out the house to be sold, everything was as good as you could expect it to be when you've split up and have a child between you.
2 years (yes, 2 years!) after he left, he met me. After around 6 months, he told his parents about me who swiftly told his ex before he had the chance to (i'm talking his mum literally text her as soon as we left their house)
she lost her shit in a big way. She stopped contact, she took the house off the market, she threatened to get someone to kill us both, she told him she would kill herself, she told him she'd move away and he'd never see their child again.
She told everyone I was the OW, including her then 8yo son.
She attacked dp in the street in front of said 8yo. She smashed the window of my car.
She has carried on acting like this for the entire duration of our relationship, they went to mediation and sorted contact etc, she tries to extort money out of us at any opportunity and pays for nothing for her own son. She has said some truly vile things over the years, and I cannot see the situation improving much, if at all.
We live quite close to her now and as such sometimes we bump into eachother, she still cannot bring herself to say hello to me, she stops and scowls and usually mutters something under her breath.
if you had told DP, or me, at the beginning of our relationship when she was unaware of me and happily co parenting with dp that she would turn into a nasty, abusive maniac who cared not a shit about her child we would have both said you were mad. But here we are.