At the end of the first season, the final shot is of J fingering his new Beretta and glaring at all the family members gathered around the pool. So definitely he has been nurturing feelings of hatred for a long, long time, all related probably to his desperate childhood.
In that case, why did he not rat the family out to the cops in Season One? Maybe because this will not get him what he actually wants...the family money and lifestyle. Sending them to jail just lands him in foster care... The reason he called Smurf in the first place was because he knew they were wealthy and because his only other option was foster care...only, maybe in California 17 is too old for that? He told Smurf in the final episode that he thought she had kept a roof over their heads...but this season has made clear that at times they were actually homeless and hungry. So Smurf apparently provided a minimum. Also: the teacher warned him not to tell the cops about anything that would implicate him. He had been involved in the two major crimes he might have been able to tell them about first hand...the watch heist where he had helped steal a car and the Pendleton heist he had helped set up by setting the fire in the building. So he really has nothing to give them. He did tell them about the things he has heard the boys talking about, but the cops were not satisfied with that info.
Next: in Season Two, he seems to warm to Smurf and tries hard to worm his way into her heart, for instance by instigating the hold up of the diner in Arizona. (But note the underlying threat when he promises her to end her life before she dies as Manny did.) But when Baz asks him what he is going to do with the "free money"($20,000) she gave each of them after the home invasion, J said, "nothing is free." Then, when working to steal Smurf's storage unit, Baz suddenly gives him fatherly advice about Smurf..."Don't want you to make my mistakes..." Oddly that all plays out in the last season.
As noted before, in the latest season, Smurf mishandles J. She takes him for granted and orders him to use his own money (and if he paid the lawyers from his own stash as well as the protection money, he would have exhausted his savings.) She ought to have directed him to sell the land in Carlsbad he later sold secretly. Then she orders him to confront the very people in the desert who had nearly killed him in the pool. She orders him to go see Lucy but forbids his use of the body armor. He sees that having POA is a terrible burden (unless he does what he did!) and the Smurf is using him just as Baz had warned him she would. He kills Morgan in a desperate effort to cover his sale of the properties.
Why does Smurf not kill him when she finds out? Maybe because a) he has just saved her life, and b) she sees he is very smart and thinks she can get him back in line and use him further. So she nullifies his actions by giving everyone property. Earlier, she tried to break up his relationship with Mia, I suppose to cut off associations outside the family. But, if she heard J's whisper in her ear, a war is about to unfold in Season 4.
Pope still has his DIY murder kit. At the end, he and J seem to be declaring war with each other. So... which side are you on, bro, which side are you on?
Smurf took out Lucy because she knew that if Pope did not kill her (Smurf) for killing Baz, Lucy would. Some people do not seem to remember all the people we know Smurf has killed or had killed: Javi's father, the guy in season one who had abandoned her mom and her, Catherine, Catherine's parents, Javi, Baz, and who knows who else. J expected her to take him out if he could not shift the blame to Morgan for the theft of the properties. She had also, for the most part, abandoned her own daughter and grandson.
Pope has not killed anyone (new) yet. He intends to kill Smurf for killing Baz, but she seems to have forestalled this with all her gaslighting.