Tall poppy syndrome. He was his father's favorite, went about in flashy clothes, and generally was the center of attention. They coveted that attention and resources, so they made him disappear and told their father that a wild animal ate him. The selling into slavery part was because they realized they could make a quick buck by selling him rather than just leaving him to die in a pit.

Whether he was obnoxious is a fair argument either way. Maybe he was an insufferable show-off, or maybe he was just that good (see how much he repeatedly succeeded in Egypt, the man was hyper-competent).

The moral of the story is how he exercised patience, restraint, and changed his backstabbing brothers' hearts by showing them the practical and emotional impact of being betrayed. Essentially he redeemed them from their wicked ways the way God seeks to redeem mankind.

The brothers were assholes.

They had different mothers. Joseph and Benjamin were from Rachel.