My Father expired a few months ago under mysterious circumstances. He was a good Muslim, very kind and loving in nature. But in financial matters he was very weak. He never took advice from knowledgeable people. He invested his life’s earnings in one project by taking a big loan by mortgaging his property. Unfortunately this business did not turn out as he expected. He was deceived by people and due to natural calamities, he suffered a great loss, and as a result he committed suicide (he left a suicide note). Since we brothers stay abroad we didn't know about the actual reason for his doing so, but during his life we knew that the land on which he was doing business had some bad Shayatin who were troubling him a lot. He could see them and so did the other villagers who stayed around. He even once asked one of them (Shayatin) what they intended and he said, “We will destroy you.” On hearing this, he became very angry and threw his slippers at them while they ran away. This went on for a while. My brother during his visit to India called upon a so-called jinn exterminator who came and performed some act and burnt the Shayatin as the people say. After this act was done a few days later my father committed suicide.
My question: after analyzing the situation what can be said of my father's death, was it natural or was it suicide? What is Islam’s ruling on such a person's death, is it Halal or Haram? Please resolve this in the light of Qur'an and Hadith.
Please guide me in asking Allah for his forgiveness and granting him a place in Jannah.

Praise be to Allah, the Lord of the Worlds; and may His blessings and peace be upon our Prophet Muhammad and upon all his Family and Companions.

Killing oneself is forbidden for any reason.  Allah Warns of severe punishment for this sin.  Allah Says (interpretation of meaning): {…And do not kill yourselves (nor kill one another). Surely, Allâh is Most Merciful to you. And whoever commits that through aggression and injustice, We shall cast him into the Fire, and that is easy for Allâh.}[4:29-30]. If a believing person faces hardship, entangled in calamities, does not have any solution for that and does not even want to live, in this case, he may make this Du'a.  "'O Allah! Let me live as long as life is better for me, and take my life if death is better for me."

By no means should a believing person commit suicide regardless of how bad his situation is.  If one does so, he is under the will of Allah; He may forgive him, or He may punish him for his sin.

Now you are advised to make Du'a for your father, ask forgiveness of Allah for him and make charity on behalf of him as much as possible.

Allah knows best.