I am watching the first four seasons over again, anticipating the fifth soon to come I hope. I'm up to the first episode of season 3 now, Tommy's wedding to Grace.

Arthur is such a colossal screw up, I am surprised he can tie his shoes without fucking it up. He has a good heart, but he is a hothead and a screw-up. The one good thing about Arthur is you can always count on him to fight, except when he is complaining about it. For example, when Tommy tells him they have to kill the Russian because he gave the wrong password, Arthur gripes and complains that he is the one who has to "shovel the shit" as he puts it. Other times he seems eager to kill. When Arthur went to get the Russian to lead him to his death, it was obvious he was up to something. He telegraphed his intentions, and the Russian picked up on it. He leads the Russian away from the other guests, and at the top of a staircase, he grabs the Russian and topples down the staircase with him. Hell, he stood as much chance hurting himself as he did the other man. He had a revolver, he could have clubbed him over the head and knocked him out, making the killing quite easy. But instead Arthur grapples with the man, allows him to pull his own pistol and Arthur nearly gets shot. Finally Arthur manages to grab his own handgun and point it at the Russian man's head, as the man pleads with him Don't, don't.

Arthur stands over him shouting "NO! NO! NO!" like an idiot before he finally shoots and kills the man.

Arthur is an emotionally unstable hothead, as likely to fuck things up by trying to improvise as he is to help a situation. He is a loyal soldier, but he is irresponsible and often unreliable. It is amazing that the family business has survived as well as it has given Arthur's unstable personality and drug problems. He is better when sober, but he is not too bright and sometimes seems to resent his younger brother getting the brains. It is dangerous to be around someone who is stupid and angry about it.

John is perhaps even more of a hothead than Arthur at times. The two of them are more of a liability than an asset at times. If Tommy were ever taken out of the picture, the other brothers would be killed off in short order.

The way John and Arthur botched the meeting with Vincente Changretta played a big part in the troubles the family faced in season four when son Luca Changretta has a vendetta against the Shelby family. Though the specific incident involved the death of the elder Changretta, John and Arthur had treated Vincente with such contempt that there was already bad blood between them.

When John beat and cut Angel Changretta, hospitalizing him, a hitman is sent after Tommy Shelby. Arthur sees him and tackles him just as he fires his handgun, hitting Grace Shelby instead of Tommy, killing her. This was precipitated by John being a hothead. John and Arthur revel in being assholes who beat and cut people at the slightest provocation, bring trouble down upon the family.
After they got Grace killed, they decided to grow a conscience, sparing Mrs. Changretta. And then Arthur killed Mr. Changretta to prevent Tommy from torturing him and interrogating him.

I was actually glad when John was killed in the vendetta in season 4. Arthur is stubborn and stupid, but has a good heart.