Are worshippers of bad character, who deal in usury, cheat, transgress against neighbors, unlawfully consume people’s wealth, and cause another group to refrain from performing the obligatory prayer, sinful?

What the questioner mentioned about some worshipers committing grave sins is a sad reality, but that does not justify abandoning prayer, because Allah Almighty says: "Indeed, prayer prohibits immorality and wrongdoing." Proper prayer motivates a Muslim to abandon evil deeds. It is authentically reported from Ibn Mas'ud: "Whoever's prayer does not command him to do good and forbid him from evil, will only increase in distance (from Allah) through it."

Therefore, it is not permissible for a Muslim to use the sins of other worshipers as an excuse to abandon prayer, for prayer is obligatory upon him, and the sins of others do not waive this obligation. It was said to the Prophet, peace and blessings be upon him: "Indeed, so-and-so prays all night, but when morning comes, he steals." He (the Prophet) said: "What you say will deter him," or he said: "His prayer will prevent him." Prayer itself atones for minor sins, and abandoning it is a greater offense than bad manners, cheating, and the like, because it is disbelief, Allah forbid.