So the snake was cheating with the dude behind his wife's back, an inherently passion-driven/impetuous thing already, and she was lying to him on top of all that, and then she has the gall to be miffed that he called her gross??
Also, her "snake tail" looks more like a big amorphous slug body in most of the frames, lol.
That's not fair. Based on the context of it, she didn't know that he was married. To her, it was a perfectly normal relationship and trusted him, which is why she tried revealing herself. When she did, he both told her that she was hideous and that he was cheating on his wife with her. I assume he said the latter because he was intending to end their relationship right there. It wasn't a "you're gross, but whatever" it was a "you're a hideous monster. who could ever love you?" The way this part is written implies that her revealing herself changed everything.
And it's not fair to say that she was lying to him. She was hiding her snake form because she had been treated like a beast her entire life. It seems she even hates herself a little bit because of the way she was born. This might be a comparison to queer people hiding their identities to protect themselves. Is it lying if a bi person doesn't tell their partner that in fear of being attacked? She had the ability to hide her tail and try to live a "normal" life. I think killing him might be a little overboard, but the author tried the sell the idea that the combination of all she had done to her and her expectations of their relationship were enough to end his life over. All the girls in prison town had to get their somehow.