Is it permissible to hack into others' devices and access their correspondence under the pretext of inviting them [to Islam] and reforming them, and then to guide people with vague stories without mentioning names, or to expose some famous writers and reformers after discovering their mistakes, which might lead to tarnishing their image and generalizing that to all preachers and reformers?

It is forbidden to pry into people's affairs, spy on them, search for their faults, or scrutinize their news, as Allah Almighty says: "O you who have believed, avoid much [negative] assumption. Indeed, some assumption is sin. And do not spy or backbite each other. Would one of you like to eat the flesh of his brother when dead? You would detest it. And fear Allah; indeed, Allah is Accepting of repentance and Merciful." [Al-Hujurat:12]. It is not permissible to spy merely based on suspicion, even if there are indications, unless there is a fear of losing a sanctity or a right that cannot be remedied. Whoever seeks out the private matters of people, Allah will expose him in his own home. This act is not excused by claiming reform or calling to good. Whoever has done this must repent, seek forgiveness, and seek absolution from those they spied on.