This is an interesting question. Fatal originates from Latin fatalis, meaning something like pertaining to fate. It is not so deadly, per se, but then if we examine what fate means, things become a little clearer:
- something that unavoidably befalls a person; fortune; lot
- the universal principle or ultimate agency by which the order of things is presumably prescribed; the decreed cause of events; time.
- that which is inevitably predetermined; destiny: Death is our ineluctable fate.
So it makes sense that fatal and fate are connected, since death is the ultimate fate of all living things.
Now fateful on the other hand, is also from the same root, but the -ful changes the meaning from something like pertaining to fate to something like characterized by fate. In other words, rather than representing our ultimate destiny, fateful describes something that has some sort of significance with regard to our fate. So fateful is not as deadly as fatal because it contributes to or furthers us toward our fate, rather than being our fate.