Hi Carina -
Thanks for providing all the background information. It helps us to see through the "clouds" in this dream, and understand its true meaning!
Fog in a dream, especially if we are driving a car, is a metaphor for "difficulty seeing the road ahead of us." In your waking life, you are torn between staying with your church, composed mostly of older people, and moving on to locate a new church with a younger congregation. The fog in the dream represents your confusion about the decision you will make.
Suddenly, your car hits something. When the fog clears, you realize you are in front of your church, and that you have killed an elderly woman from your congregation. It's a terrible accident, but its meaning is much different than it appears.
Death in dreams is a symbol for change and separation, and should not be interpreted literally. Accordingly, your dream doesn't mean that you ever would harm a church member. To the contrary, you are worried about the members of your congregation being hurt -- by your actions -- if you decide to leave the church.
If you and your fiancé decide to move on in your lives, your hearts should rest assured that no member of your congregation will begrudge you for choosing to explore your magical new lives together. Many years ago, the members of your congregation also stood in your shoes, and gave life the youthful embrace it deserves. They will miss you when you choose to leave, but they also will give you their blessing.