The popular theory is that Sunder Rajan committed sacrilege by opening vaults of God and the sarpa dosham killed him. My theory is more along the lines of CBI officer Lakshmi Narayanan (played by SPB) in the world famous movie Thiruda Thiruda. Where he tells all of Thirunelveli that 1000 Cores of money is hid in a Container Lorry somewhere in the Taluk. When "thirudan 100 mile thoorathila irundhale naadi thudikkum" Inspector Santhosam asks Lakshmi Narayanan why he had to disclose such a huge secret when the CBI could have searched for it in stealth. Lakshmi Narayanan's theory is "ooru rendu patta CBI kaaranukku kondattam" which means the 1000 crores will not be smuggled out of the village under the watchful eyes of thousands of villagers.
He may have died because he opened the vault.If I were to write a story around this, it would have Sunder Rajan knowingly risking his life for the greater good.
Also, while the demand by small section of people that this money be used for development/charitable purposes is stand-alone stupid. Let us set that aside. Is it that hard to figure out that (a) majority of public are satisfied once the money goes to *a* charity and don't care what the charity does with it and (b) most charities are a front end for a "different" kind of organization ?