The Summoner was a church official who was responsible for calling out sinners before the courts. Although this was his job, he himself is a sinner. The Summoner would allow a sinner to keep a mistress for an entire year just in return for a quart of wine. He is sympathetic to such people because in all likelihood he commits the same sin himself.
Locke is trying to say that tyranny is the exercise of power for the wrong reason, basically meaning that it is abusing ones power over someone or something
