Assalaamu alaykum. I unintentionally said that a man (who was the president of India) was born by mistake. Later, I realized that this sentence is not correct because Allah gave him life. I said that only because I hate that person. Does it constitute disbelief to say something like that?

All perfect praise be to Allah, the Lord of the worlds. I testify that there is none worthy of worship except Allah and that Muhammad is His slave and Messenger. 

There is no doubt that this statement is incorrect; rather, it is very serious because its apparent meaning challenges the wisdom of Allah and His Order. This is Kufr (disbelief), but you did not mean this evil meaning. Rather, you meant to express your hatred for him. Therefore, this does not mean that you committed Kufr. However, you must restrain your tongue from pronouncing such a statement, as taking this lightly might lead to evil consequences.

Abu Hurayrah narrated that the Prophet said, "A person may say a word which pleases Allah without giving it much importance, and because of it Allah raises him to degrees (of reward); on the other hand, a person may say a word which displeases Allah without giving it much importance, and because of it, he will be thrown into Hellfire." [Al-Bukhaari]

Allah knows best.