Could preoccupation with death/ suicide be an outgrowth of depression; or could it be prenatal?

Doctors now understand that a preoccupation with death and/or thoughts of suicide are symptoms of clinical depression. That doesn't mean that all people who have depression will have these symptoms. Not all depression is equally severe.

Bad events don't "label" someone "accident prone". People are the ones who label themselves or others "accident prone". A three-year-old who is has the normal curiosity of a preschooler and puts a hairpin into an outlet has not had an accident. He is living in a house that is not "child-proofed" and where he has access to things like hairpins. This incident is not an accident.

Some people do have a lot of accidents, but that's usually because they move too quickly for their own level of physical grace; or because they take careless chances.

I think the individual who felt the need for anti-depressants at 20 was a generally unhappy individual (either for reasons associated with his life and life circumstances, or because he had a chemical imbalance that made him unhappy). If later he continues to feel as if he wishes he were never born, I think he still needs some kind of professional help (either with counseling or medication) and/or he needs some change in his life that will make him feel happier.

People with chronically difficult/stressful lives can eventually develop depression, but that's not the only reason people become depressed. Some people have chemical imbalances. Some have emotional issues that contribute to their feeling unhappy over long periods of time.