i've been rewatching the part of the 1967 episodes where barnabas,convinced that david knows his "secret",decided that the boy needed to be done away with.
i had forgotten how much energy they had given to this little subplot.it's really quite shocking.
self-preservation is his modus operandi during this part of the show but still killing a child and a member of your own family is pretty harsh.this deed might spare barnabas exposure in the short-term(if david did indeed know what he was)but in the end would have thwarted alot of his bigger goals.for example.
the police:as julia rightly pointed out if david disappeared the police would conduct an investigation of the likes the town had never seen.it's one thing when a waitress and the daughter of the town drunk turns up missing but when it's the heir to the town's most prominent family it's quite another.they would search the old house top-to-bottom.no amount of stalling and double talk from willie would keep them out.
vicki:supposedly barnabas is in love with her.but if david died she would be emotionally destraught.inconsolable.at the very least the collins family would no longer need the services of a governess and she would leave.
elizabeth:it's made pretty clear that liz keeps the house and the business afloat for david's sake.she expects him to carry on the family name.neither roger or carolyn seem to have the inclination to take over the whole thing so she might have decided to sell the whole thing off...including the old house.
julia:she always said her arrangment with barnabas did not include the taking of a life.if david's killing did in fact take place she would have ratted him out anyways and this all would have been for naught.