Don John hates that his brother Don Pedro has more control over him due to Pedro being wealthier. Don Pedro in the play is always judge him and make him seem happier instead of being jealous and depressed.
HERO IS CHEATING ON YOU!!!
Don John gets the Idea of how to get control over the town. He has to ruin the love between Claudio and Hero . Because of that the town will be in crisis. Don John will have power over the town because he can make things better.
He would make this happen by having two people act as Hero and a different Man making love. People would hear their names and instantly spread the news.
Claudio would find out and the whole town would be angry about it. Yet Don Pedro is the only one who knows that Don Jon is up to this. So don john failed because don Pedro told everyone.
So don john failed because don Pedro told everyone. Every one got angry at don John for breaking the wedding up.
Because he attempted to get control over the town and breaking up the marrige he got thrown in the dungeon.