Am I wrong for thinking my ex is an asshole?

Okay so we recently broke up August 1st. So that's three months ago today but my dumbass said we could still be friends. At first, I was contacting him constantly because I wasn't over him or the relationship but then I got an epiphany and let that shit go. Then he starts contacting me almost like he used to before the breakup and I'm thinking now we can finally start to be friends. Because there's many exes that remain friends after a breakup.

Nope nope nope. Wrong. He recently told me that he's in love with his best friend from high school whose married with a baby. They briefly dated back in high school but it went nowhere. Now I'm wondering if he had these feelings while we were together. He said he didn't but I don't know if I believe that. But she also has feelings for him since her marriage is falling apart.

He sought advice from me and I wanted to tell him to go f himself but instead I was supportive and said he should follow his heart despite the fucked up situation.

So am I the asshole for wanting to tell him to go f himself? I'm conflicted because he's always been my best friend since college which was over 10 years ago but at the same time I'm like why tell me something so fucked up. Like are you deliberately trying to hurt me?

He did apologize and said he knows it's not fair to me but I still want to say f you so bad.

And I know I mentioned this in my last question but I'm just so flabbergasted that he would do this and so soon after the breakup.
Am I wrong for thinking my ex is an asshole?
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