What is the advice for someone whose main preoccupation is talking about the flaws of Muslims and non-Muslims, and that they are astray, without discussing how to guide them?

Calling to Allah should be done with kindness, wisdom, and insight, as Allah Almighty says: (Invite to the way of your Lord with wisdom and good instruction, and argue with them in a way that is best). A Muslim's preoccupation with mentioning the faults of others is blameworthy, as it may distract him from his own faults. He should ask Allah for guidance for people and strive to guide them without falling into slandering them or seeking out their hidden faults, unless there is a legitimate Sharia interest, such as warning.