If you go to the Holy Quran you see that cursing people who are the opponents of the truth and do not want to accept what is clear, they are cursed not only by the prophets but by the angels and God also.Lanat is basically losing the blessings of God. It is only invoked after all argumentation has been presented and the opponents still refuse to knowledge the truth.
First some verses from the Qur’an:
How shall Allah guide a people who have disbelieved after believing and who had borne witness that the Messenger was true and to whom clear proofs had come? And Allah guides not the wrongdoing people. Of such the reward is that on them shall be the curse of Allah and of angels and of men, all together.(Chapter 3 verses 87-88)
Allah is speaking here that the curse of not just Allah alone will be on them but also the angels and the men in response to the rejection of truth.
Allah again States:
Now whoso disputes with thee concerning him, after what has come to thee of knowledge, say to him, ‘Come, let us call our sons and your sons, and our women and your women, and our people and your people; then let us pray fervently and invoke the curse of Allah on those who lie.’(Chapter 3 verse 62)
The questioners never mention why the 1000 curses were mentioned. If he had read the book he would have understood why. This is a book called Nurul Haq in response to a book by Imam ud din who was an ex Muslim and became a Christian and was so abusive to Muhammad (Saw) and Islam that even Hindus said this is unacceptable and that if there would be another uprising in India this book would be the reason. Ahmad(As) said to stop abusing Muhammad(saw) and that you criticize the Quran, you can never match it. In fact you cannot even compare with me.
Ahmad(As) said if you can write something better then nurul Haq ill give you 5000 rupees but you will never come forward. Therefore if they don’t come forward and continue the abuse, then may 1000 curses be upon you. All out of love for Islam and Muhammad(saw).And all Muslims were to say ameen to each lanat. Nothing wrong with this at all. All out of love for Muhammad(saw) and Islam.