The tradition is a few centuries old.
Half-staff (or, on a ship, half-mast).
To honor, remember, reflect on a death.
The death of a warrior.
The death of a famed politician.
The death of other heroes in the line of duty.
(Don't crucify me) People, regardless of age, who are murdered?
Lowering (after raising, which is proper) the flag to honor a life should be reserved for those who die in service to their country. Personally, I'm not convinced a politician (regardless of whether I like him/her or not) isn't, well, "worthy," of the honor.
A soldier who dies in battle? Certainly (and primarily)
A law official who is killed in the line-of-duty? Absolutely
Firefighters or other firstresponders who lose their lives in active service? Duh
But not an actor....not those who happen to be in wrong place at wrong time....
Counter rant if you wish....