I think his behaviour relating to the betrayal matters almost as much as the betrayal itself.
I know this list of questions belongs in some kind of flowchart or magazine quiz with points at the end, but it might be useful (and it's not at all intended as stuff to be answered here; that would be an interrogation).
Did he tell you, or did you find out for yourself (or hear from someone else)?
If you did find out by yourself, how did he react when you confronted him? Did he come clean right away, or did he gaslight you, drag you through exhausting discussions and only admit everything when you produced absolute proof?
Did he try to blame anything that had happened on you, either directly or indirectly? Did he insinuate that perhaps it was your lack of trust/affection/some other bullshit that drove him to this?
Did he say that you were GLAD he'd fucked up, because after all that's what you EXPECT so CONGRATULATIONS, you've WON? I bet you're loving this.
Did he immediately counter with a list of all your mistakes and crap behaviour; stuff which apparently didn't bear mentioning until this exact moment when he's been caught out?
Was he spiteful? Verbally abusive? Were you worried about your safety?
Did he have ideas how to resolve the situation, or did he ask 'well what do you want me to say/do?', over and over in a sullen, resigned tone.
How many times did he lie to your face about this issue? Not offer partial information, not vaguely mislead: times you've asked about X and received a deliberate lie?
If you were ever concerned about X before, and said so, did he laugh at your concerns or get nastily defensive?
After telling you (or realising you'd found out), did he quickly shift the focus onto his own emotional and mental health? Are you supposed to be worried about him right now?
Have his actions put you or the DC at possible risk? What tangible negative consequences have his actions had on you, and does he fully recognise this?
How much conscious thought, effort and calculation was involved in this betrayal? Was it one really stupid decision, then a messy aftermath of trying to hide and fix things but only making them worse? Or was it something sustained and systemic?
So many questions... but, IMO, bad decisions are one thing and bad character is another. Bad character shows itself in more than just the betrayal.
My friend's partner cheated for a long, long time, got caught out, made it all about his emotions (hinting at suicide), blamed her (made him feel inadequate), launched a massive campaign to romance her back. Then did the exact same thing a decade later. That is some crappy character.