Am I Wrong for Not Returning Money My Ex-husband Sent to Me Mistakenly?

I have found myself in a bit of a dilemma, and I know that money can be a difficult conversation to have. But sometimes the discomfort can be used as an excuse not to hold people accountable. In my situation, I had almost given up on the money that my ex-husband owed me until it found a way to me unexpectedly.

I work as a nurse, so my working times can be really extreme, especially depending on the number of shifts I am working. On this particular day, I had worked a double shift and could not wait to get into bed and pass out.
My four children were also with their grandparents, so my plan was to sleep through half of the day. But while I was fast asleep, my ex-husband mistakenly made two payment transfers of $8,700 and $9,275. According to the notification, the money was reflected at around 07:40 a.m., but I did not notice because I had just come from my long shift.

By the look of things, he realized his mistake at about 08:30, but by then I was dead asleep. I woke up at 4:00 p.m. and was met with hundreds of missed calls and about 150 text messages begging me to send the money back.
The frantic messages did not make sense because I had just woken up and barely knew my name. My initial response was to send the money back, but in the process of waking up, I came to my senses and remembered that my ex-husband owed me money.

For more context on this money dilemma, my ex-husband and I have been divorced for seven years. He is the father to all my four children, but four years ago, he faced a few financial setbacks when he lost his job and fell back on his rent.
At the time he asked for the money, he had come up with a payment plan, but he never stuck to it. I still have those text messages, which will serve as my legal trail. This money is separate from his child support payments, those are handled by the court, and I would like to have them not get mixed up with this transaction because it was just between the two of us.

But after so many years of promising to pay it back and failing to do so, while living his best life with his girlfriend, I saw the accidental money transfer as an opportunity to pay myself back.
I kept the money he owed me and sent back the rest, but since then, I have been dealing with insulting text messages. The main frustration is that he hoped to use the money to buy his girlfriend an engagement ring. He planned on popping the question on the coming weekend, so my taking the money he owed ruined the surprise.

Naturally, this painted me as a villain in their story because the girlfriend had already been bad-mouthing me so much that it was borderline harassment, which led me to shut down my social media. This event impacted the cordial relationship I had with my ex-husband’s parents.

After all the attacks I had been receiving from my ex, continuously telling me how selfish and self-centered I was, and that I did not care for his happiness I wondered if I was wrong. He even went as far as saying that I was jealous that his relationship was progressing and that I still held resentment from our divorce.

This is not true. We have been separated for almost a decade, and my life has changed significantly since then. I am very invested in keeping my children safe and making sure they are happy. My problem with my ex-husband is that he never considered paying me back when I was trying to support him in his time of need.
So to call me selfish and jealous seems like a slap in the face. It gives me the impression that he does not recognize the support I have given him. I also fully understand that as a parent, whatever financial hurdles we are experiencing will impact our children, so my loaning that money was for the sake of maintaining a healthy co-parent dynamic.

Nonetheless, I decided to share the story with a colleague to understand how the situation looked from someone else’s perspective. While I was telling the story, another person from work overheard and decided I was wrong for taking the money owed to me.
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I am aware that the legal trail I have of him committing to a payment plan would have given me enough evidence to sue him. But honestly, that entire process would have cost me more than he owed me.
The child support company is also aware of the many payments in arrears, and that is being handled. I think it has even gotten to a point where his wages will be implicated. Even if my ex had taken this to court, I think I still would have won because I believe I did the right thing by only taking the money he owed me.

But now, after my ex’s reaction and my colleagues, I am wondering if I was wrong for taking the money, and I am open to knowing what everyone thinks and if there would have been a better way to deal with the situation.