When should we frighten people?

Imam Abu Al Faraj ibn Al Jawzy who died in (597AH ) saidin his book "Sayd Al Khatir " :

The best thing is to be moderate in everything . So, when we (the scholars) see those who seek the world whose hopes hang with it, their bad deeds overcome good deeds, forget the meeting of Allah, and dare to fall into sins and evils, we are commanded to remind them with death, graves, and the Hereafter.However, if man always remebers death and the events of the Hereafter, over reminind him of death will be of no avail, but rather could lead to reluctance from actions (i.e., he may get sick by depression).
 

He adds: Man who always fears Allah (Glory be to Him) and oft-remembering of the Hereafter to busy himself with something else in order not to recall death, to have a hope in life, to write books, and do righteous deeds. However, if he keeps recalling death, the corruption shall be greater than the benefit. Did not you hear that the Prophet raced `A'ishah (may Allah be pleased with her) once and she raced him then he raced her and he won, and he used to go out to busy himself with something else?

Frequent looking at the reality of things corrupts the body and annoys the soul (i.e., afflicts the body with physical and psychological diseases).

Imam Ahmad ibn Hanbal (may Allah bestow mercy on his soul) reported that he asked Allah (Glory be to Him) to open for him the gate of fear, thereupon his request was granted, then he feared for his mind and asked Allah to screen it from him.

Ponder over this origin because man has to disobey his soul (When fear possesses it, the balance which is made for the construction of earth will be lost) therefore in this case disobedience of one's soul is required.

End of the speech of Al Jawzy with slight paraphrasing.

I recall this Hadith which At-Tirmidhy and Ibn Majah (may Allah bestow mercy on their souls) reported on the authority of Abu Sa`id Al Khudry (may Allah be pleased with him) that the Messenger of Allah (peace be upon him) said: "When you go to a patient, pray for his long life. That, indeed, does not avert the decree in any way but, of course, it cheers up the patient ."

Ibn Al Athir (may Allah bestow mercy on his soul) said on his commentary on the Hadith: Pray for his long life means give him hope and ask Allah to longen his life
.

Indeed, moderation and balance are required, and the meaning of wisdom is to put everything in its proper place.

I attended a wedding cermony, during which one of young people was asked to deliver a speech, so he delivered a speech about the horrors of the Day of Recompense. Thereafter, I got out without tasting food.

Someone told me that he was at a wedding cermony and when the contract was signed and congratulations showered upon the couples, one of the attendants asked someone of nice voice to recite some Ayahs from the Qur'an. The young man started to recite: "The divorce is twice, after that, either you retain her on reasonable terms or release her with kindness . " Although I doubt this narration, I benefit from its significance.

From my personal experience, I believe that children and the students of the primary and prep school should be pushed nicely to do the required actions then they should be granted a small portion of fear gradually to make the required balance. Allah knows the best.