Something people really need to start understanding about Joe’s anger is that is comes from a place of heartbreak. Anger manifests in him when he is hurt deeply and personally.
Joe is soft, Joe is kind, Joe is not the type of man that becomes angry with mild or petty provocation, and he is a very forgiving man, as his relationship with Nicky, - a man who once hated him and and only came to meet him by virtue of a quest against Joe’s people - shows.
He doesn’t come to anger at the drop of a hat, or hold grudges. (no don’t @ me, he doesn’t hold grudges)
When Booker’s betrayal is revealed in the lab and when Nicky is shot - those moments are both heartbreaking and the key to his anger, they unlock it, unlock something that’s held deep down within the man. Anger, or rather true anger does not come easily to Joe, yes, we see him experience, irritation, annoyance, a surge of pique, a sharp and impulsive surge of surface anger, but all these instances differ visibly from his real anger. Example: when he headbutts Merrick versus him killing Keane, the former is almost showy, it’s an action spurred by indignation, it’s the defiant smirk of a loyal operative - that has been captured by the enemy - before the start of an interrogation. The latter action is executed with jaded quiet, there is no posturing, no grand defiance for the benefit of the room, there is only the fact that Joe has been hurt, and nothing else.
Joe’s anger is kept side by side with his love - not his loves, his love - and it emerges to protect his or avenge his love when it is damaged.
Compared to his teammate’s anger:
Nicky’s anger is righteous anger, it’s always running and just under his skin, tidily stored away and impeccably controlled, yes, but it’s always there, ready to snake out at the slightest hint of a threat.
Andy wears her anger like a garment, and much like a garment’s fabric it swirls and ripples and sometimes catches and hits other people with its folds.
Booker? Well, we never once see him display anger, do we? And he might not be capable of anger anymore, anger is one of the first things to go when apathy sets in.
Joe’s anger is stored deep, and stays deep, and never emerges except for when he is truly, truly hurt. And one of the only ways to hurt this man is through his heart, through his love. He’s strong, he’s a warrior, a legend, a hero, he can take, and has taken everything the world has thrown at him. So you hurt him by striking at what has kept him buoyant through the long years, which is his where his care is placed.
His family, his lover, Booker struck at his family, their family, Keane struck at his love, and both of these things rattled Joe to the core, you know they did. Such an attack would both break his heart and anger him.
It makes what he said to Keane more meaningful. Because while Joe’s kindness and love has become a thing of legend, beware if you ever threaten it. Beware if you break his heart. Beware if you betray him. You should not do that.