Jax progressively going downhill
There were a few times in the series when Jax suffers crises which send him deeper into his dark side.
Over the course of the series Jax loses much of his good nature, becomes more and more hardened, until the beginning of season 7 when he abandons his vision for the club and can only see red in his need for revenge. In the first season he is very reticent to deliver a coup de gras to the head of a Mayan who had just shot him. Jax instantly shot him back in self defense and the Mayan went down, badly injured. Jax was wearing a vest and was only bruised.
Clay wanted Jax to shoot him again to finish him off and Jax clearly didn't want to do that. Self defense is one thing but killing a man who is not an immediate threat was quite another. He died quickly from the first shot so the issue was resolved without anyone needing to finish him off.
By the time he took the President's patch from Clay Jax was very comfortable with killing, and after Tara died he became very cold blooded, killing without compunction.
When Jax figured out that Clay had ordered Opie's death and killed Opie's wife he started to become angry all the time, understandably. Clay screwed up over and over and Jax steadily became more and more distrustful and disillusioned.
Opie's death hit Jax very hard as well. Opie helped keep Jax level, and they had a shared vision for how the club should be run so there was less exposure to threat of arrest or death.
I think another seminal event was when Jax found out that Clay had ordered Tara's death.
The biggest event of course was Tara's death. Jax was already comfortable with killing people. After Tara died though, Jax could only think about killing the Chinese. His plan to kill off their business and work his way into their trust didn't work from the start. When he killed off the Chinese and stole their heroin, he was the prime suspect and the Chinese began to distrust the MC.
He would have been smarter to listen to August Marks and wait, gather better intel. and make better plans. But he was seeing red.
Though Jax was a better person than Gemma, in certain ways he had become like her. He was intensely emotional and loved his wife and children fiercely, like Gemma. When gripped by deep emotion his decision making was faulty, like Gemma.
The biggest difference between Jax and Gemma, to me, is that Gemma to hid her mistakes, like killing Tara, while Jax admitted his mistakes (unfairly killing Jerry, and all the killings as a result of believing Gemma's lie), even when admitting them meant that he would get the mayhem vote. Gemma did everything she could to avoid anyone finding out about her mistakes.
I know I didn't recall all the seminal events which affected him. What do you guys think?