Choosing not to profit off of fossil fuels may be a 'poor decision' from a financial perspective, but it's a good moral decision. The fact that our financial systems incentivize poor moral behavior makes them poor systems.
It seems strange to me to cast everyone with an index fund as morally corrupt.
In order to profit from them you have to have some form of investment in them, which means in a small way you have contributed to the cause of promoting fossil fuel usage.
Exactly how morally corrupt people with an index fund are is a question I'll leave for philosophers.