Would "stand your ground" laws and the "castle doctrine" apply to shooting trick or treaters?

No in California.

Yes in Florida.

No. A child ringing a doorbell and saying Trick or treat would not be considered a threat to life and limb.

Frm Wiki
“Stand-your-ground law is a law that authorizes a person to protect and defend one’s own life and limb against threat or perceived threat. This law states that an individual has no duty to retreat from any place he/she has a lawful right to be and may use any level of force, including lethal, if he/she reasonably believes he/she faces an imminent and immediate threat of serious bodily harm or death; this is as opposed to duty to retreat laws.
Forty-five U.S. states and the U.S. territory of Guam have adopted the castle doctrine,[1] stating that a person has no duty to retreat when his/her home is attacked.”

You may, however, shoot them for sport as gun season opens 10/15 in most states.

No, not even in Florida.

It’s not against the law for anyone to knock on your front door.

If they entered say, your garage, there’s a better argument. But definitely not knocking at the front door.

I suppose “trick or treat” is an implied threat.

no

Boy has the bar lowered so much, wonder if this question would fly if it said shoot women, gays, Jews, or Mexicans?

Only if those Black ones were trying to invade your house, otherwise it might be a hate crime…..question is, ‘Do you feel lucky, well do ya redacted?’ (in Clint Eastwood voice)

Depends where. Death of Yoshihiro Hattori

Verdict – Not guilty

You’d probably be ok in South Africa. Hell, they barely even punish people who murder their girlfriends when they go to take a piss.

@LuckyGuy ”...perceived threat…”

So yes, you can blast children dressed as monsters in Florida.