You can't forgive someone who hasn't hurt you, or ask for forgiveness from someone you haven't hurt. Even in a situation where there's current suffering as a long-term result of some long-dead people's actions, any remaining problems today are not caused by the long-dead culprits but by people today who abuse the situation for advantage.
Take the slave trade as an example. Underachievement by black students and higher imprisonment rates amongst blacks aren't the fault of the original slavers. Instead, they're the fault of those biased whites who consciously or unconsciously victimise blacks, or of those blacks who incite criminal behaviour as role models. The causes of both can be traced back to the slavers, but the people doing the damage today are the guilty ones today, and as such are the only ones that someone today has a right to forgive. Forgiving slavery and asking forgiveness for slavery are both impossible, because none of us alive today were involved on either side.
So back to Judas, we can't forgive someone who's already dead. The purpose of teaching about Judas's actions is not to forgive or damn, it's for them to be understood as something to be avoided. Peter and Thomas are the same. Judas teaches not to betray your friends for gain; Peter teaches to hold faith with your friends even when it's dangerous; and Thomas teaches to trust what your friends tell you.