Why do people get fixated on the affair partner, but not their partner?

[PsPsPscat]

Just why? Is it easier to direct the anger at an outsider than at your actual partner and hold them responsible?
My ex got assaulted and even briefly hospitalised after the husband of a woman he was seeing found out about them. I absolutely understand the anger and frustration at this, but I can't fathom someone focussing this much anger on the affair partner, rather than the cheating spouse?
Before anyone has a go at me: No, I would absolutely not want to see this level of aggression being aimed at a woman and that's not what I'm suggesting. But I have seen the messaged that were shared before the attack and they were unhinged and completely distorted, and it made me wonder what on earth makes people so blind with anger that they lose all reason.

Not my family, but someone I know cheated on his wife and got together with the affair partner after his wife died. He rightly said the reason people can forgive him and not his affair partner is because we all have a history with him other than the affair. The only thing we know about his partner is that she had an affair with a married man for years and destroyed his wife's wellbeing for a really long time. So maybe that's the thing.

Logically, the married partner owes their spouse more loyalty than the affair partner.

I actually don’t know anyone who has blamed the affair partner tbh.

I do.
It's easier. Blame the other person and then your unit is safe, nothing needs to happen.

It’s easier to make a villain out of someone you don’t know?

I was furious with both of them, to be honest. I used to fantasise a lot about beating the crap out of both of them but didn't actually do anything. I came very close to giving my ex a damn good crack on the shin with his cricket bat when he came to get his stuff and asked me to pass It to him. But I took a couple of deep breaths and didn't because I'm not a violent person really. They hurt me so badly the impulse was to lash out and hurt them back. I guess some people have poor impulse control.

The affair definitely hit my mental health badly for a long time and I hold them equally responsible for what they both knowingly chose to do to me.

I absolutely understand the anger. But I saw the messages and he seemed to completely fixate on the affair partner being the driving force despite his wife being the far pushier one (according to the messages). This is what made me wonder. Not condoning someone seeing anyone who is in a relationship but this seems so extreme.

I was mad at both. My DP betrayed me, and the OH knew I existed and still continued with the affair anyway. She could have said no. Apparently, she started it anyway.
Both were to blame.

It's because they have a modicum of love invested in that person still. Rightly or wrongly. They find it hard to be fully angry at someone they still love, at least a little bit. Ask me how I know!!!

I think with men they're furious that another man has taken whats theirs. They see it as a massive insult to their masculinity so go out to defend their territory with violence.
When women blame the OW I think its often because they need a way to absolve their husband because they want to forgive him and continue the relationship. It helps to see him as led astray by another woman. Also there is a higher bar for women, its like they feel that men have less control so other women should be 'sisterly' and not tempt them.

Well to be direct, they were chosen over you - you might see it as they are everything you’re not and you’re inferior which is rage inducing