I think it's completely normal and human to feel negatively towards the AP. That said, I don't think the AP is ever more to blame than the spouse. The AP may be morally bankrupt and/or predatory, but they were not the ones who vowed to be faithful to you. That doesn't absolve them from basic human decency and respect for others marriages and relationships, but there are always going to be selfish others with poor boundaries. The onus is on the spouse, as the one who took the vow, to keep their promises. IMHO, if one is playing the blame game, vows incur higher culpability.
That aside, although negative feelings might be normal and to some extent unavoidable, focusing on them does more harm then good. At minimum, you are giving way too much space in your head for someone who does not deserve to be there.
