What’s something you’d consider unforgivable?

I'd say corruption (public or private).

I think: hypocrisy. I cannot live with people, who are not authentic!!!

Without hesitation I'd say both "violence" and "abuse". Any kind and sort. These are two "ignominies" in my opinion, let alone being them unforgivable.

Being a double agent

selfish
double standard

something that's unforgivable today won't be tomorrow for example too

Refusing to offer me a cake. Not only it is unforgivable, but it needs retaliation é_è

Cheating, breaking too many promises, acting interested then saying you aren't interested, breaking someone's heart, etc.

Refusing to offer me a cake. Not only it is unforgivable, but it needs retaliation é_è

Violence, cowardice, betrayal, harassment, (serious) insults, blackmailing...
Those are unforgivable for me