Qunut should be said at times of calamity with a supplication that is appropriate, not with Du`a al-Qunut
Question

At times of calamity, some imams say Qunut asking for forgiveness and mercy, or they recite Du`a al-Qunut  (“Allaahumma ihdina fiman hadayta/O Allaah, guide us among those whom You have guided..”). What is prescribed in Islam in such cases?.
Praise be to Allah.What Islam prescribes for Qunut at times of calamity is that the supplication should be appropriate to the calamity, and that Du`a al-Qunut (“Allahumma ihdina fiman hadayta/O Allah, guide us and among those whom You have guided..”) should not be recited.
The Prophet (blessings and peace of Allah be upon him) offered supplication (du`a) at times of calamity on a number of occasions, and his supplication was appropriate to the calamity for which he was praying Qunut. When he prayed for the salvation of the weak and oppressed in Makkah, he said: “O Allah, O Allah, save al-Waleed ibn al-Waleed, Salamah ibn Hishaam, ‘Ayyaash ibn Abi Rabee’ah and the weak and oppressed believers.” When he prayed against some of the Arab tribes because of the severity of their enmity towards Allah and His Messenger, he said: “O Allah, punish Mudar severely and send upon them a famine like that of Yoosuf.” Narrated by Muslim (675).
When he (blessings of Allah be upon him) prayed against some of the Arab tribes who had betrayed 70 of the Sahaabah and killed them, he prayed that they be cursed. It was narrated that Ibn ‘Abbaas (may Allah be pleased with him) said: The Messenger of Allah (blessings and peace of Allah be upon him) prayed Qunut for a month in Zuhr, ‘Asr, Maghrib, ‘Isha’ and Fajr prayer, when he said: Sami’a Allahu liman hamidah in the last rak’ah, praying against some tribes of Banu Sulaym, and against Ri’l, Dhakwaan and ‘Usayyah, and those who were behind him said Ameen. Narrated by Abu Dawood (1443). Ibn al-Qayyim said: A saheeh hadeeth. Zaad al-Ma’aad, 1/280; classed as hasan by al-Albaani in Saheeh Abi Dawood.
According to a report narrated by Muslim (679), he said: “O Allah, curse Banu Lihyaan, Ri’l, Dhakwaan and ‘Usayyah, for they have disobeyed Allah and His Messenger.”
According to a report narrated by al-Bukhari (4070) from Ibn ‘Umar (may Allah be pleased with him), he [Ibn ‘Umar] heard the Messenger of Allah (blessings and peace of Allah be upon him) say, when he lifted his head from bowing in the second rak’ah of Fajr: “O Allah, curse So and so, and So and so, and So and so,” after he said Sami’a Allahu liman hamidah, Rabbana wa laka’l-hamd.
Shaykh al-Islam Ibn Taymiyah (may Allah have mercy on him) said:
It is prescribed to say Qunut at times of calamity, praying for the believers and praying against the disbelievers, in Fajr and other prayers. Thus ‘Umar said Qunut when fighting the Christians, in his du`a in which he said: “O Allah, curse the disbelievers of the people of the Book…” Similarly, ‘Ali (may Allah be pleased with him) once fought a people and said Qunut and prayed against them. The person who is saying Qunut should say, at the time of each calamity, a supplication that is appropriate to that calamity. If he names the believers for whom he is praying and the disbelievers against whom he is praying, that is good.
Majmoo’ al-Fatawa (22/271).
It says in Fatawa al-Lajnah al-Daa’imah (22/271):
It is proven that the Prophet (blessings and peace of Allah be upon him) used to say Qunut at times of calamity, praying against the aggressors among the disbelievers, and praying for the week and oppressed Muslims to be saved from the plots and captivity of the kaafirs. End quote.
Shaykh Ibn ‘Uthaymeen said in al-Sharh al-Mumti’ (4/45): He should say Qunut with a supplication that is appropriate to the calamity that has come. Hence the Messenger (blessings and peace of Allah be upon him) used to say Qunut with words that were appropriate to the calamity, and he did not say “Allahumma ihdini fiman hadayta/O Allah, guide me among those whom You have guided…” as some of the common folk do. It is not narrated in any hadeeth, saheeh or da’eef, that the Messenger (blessings and peace of Allah be upon him) ever used to say “Allahumma ihdini fiman hadayta/O Allah, guide me among those whom You have guided…”) in obligatory prayers; rather he would offer a supplication that was appropriate to that calamity. On one occasion, he (blessings and peace of Allah be upon him) prayed for some of the weak and oppressed people, asking Allah to save them, until they came (to him in Madeenah). End quote.
Based on this, the worshipper should choose a supplication that is appropriate to the calamity, and say that.
Whoever says the du`a of the Prophet (blessings and peace of Allah be upon him) because it is appropriate to a calamity that has befallen the Muslim, such as saying with regard to that which has befallen us nowadays, “O Allah, save the weak and oppressed believers in Gaza; O Allah, help them; O Allah, punish the Jews and Christians, and those who support and help them, severely; O Allah, curse them; O Allah, send upon them years like the years of Yoosuf,” has done well, because the supplication of the Prophet (blessings and peace of Allah be upon him) is the best and most comprehensive supplication that can be offered.
And Allah knows best.