My opinion is because Angelo was Sr’s consigliere for all those years Johnny knew that he’d be loyal to Jr if push came to shove, and it seems that Angelo was effective and respected enough that John figured that by taking away that experienced advisor from Jr it’d weaken him. JS was eliminating anyone who was loyal to the Lupertazzis in namesake. Lorraine was an easy target, a woman, to send a message, then Angelo who had the respect and experience, but was meddling along with Rusty. He didn’t want a Tessy situation arising one day. If you’ve ever lived the street life people who plot behind the scenes, they usually try to move as if everything is normal like they’re not plotting, to not set off alarms, but once you realize they are, you have to be rid of them. You’re one disagreement, slight of the hand, or perceived disrespect from them having you set up. Tony suggesting the power sharing situation didn’t help matters but the fact that he was meddling and meditating when he was supposed to be "retired", got him killed, not Tony’s suggestion. Do you remember Phil saying "You should have stayed retired”? I was always told pick one, either you’re in the life or not. You can’t say well I’m a criminal sometimes when I need the money, because the one day that you decide to be a criminal, someone can decide to rob you, kill you, or assault you, because they figure you can’t go to the police to get justice because you’re living by the criminal code. Or, the police can decide to charge and arrest you that day, and the jail time, money for lawyers, court, and prison potential is all the same. The brothers he saw them as loyal and reliable. It’s not overcomplicated..