No,I think all the texts were sent and were horrible, though the context is still that she says she believes he had been lying and cheating. In that context she seems to have been in a spiral of anger, and want to hurt him as she feels hurt. No, not justification, just context. Many people act awfully in the midst of anger. Often in these situations men act violently towards women, and sometimes vice versa. I think AE often denigrated IG in the marriage and if so that's abusive and wrong. I think AE will claim that he behaved similarly towards her.
I think the threats she issued were completely wrong and abusive. If she said things like she would lie about him abusing her then this is wrong. If he lied to his wife about having an affair that's wrong too imho. I'm not suggesting these are equivalents though both are based on dishonesty.
I wondered if the majority here think that IG was unhappy from near the beginning? He must have been in love with her at some point no? It seems a shame that no marriage counselling was tried a long time ago.