Avoidance of Greater Evil

BB and CC, both armed with knives, attacked FT. The victim's son, ST, upon seeing the attack, drew his gun but was prevented from shooting the attackers by AA, who grappled with him for possession of the gun.

FT died from knife wounds. AA, BB, and CC were charged with murder.

In his defense, AA invoked the justifying circumstance of avoidance of greater evil or injury, contending that by preventing ST from shooting BB and CC, he merely avoided a greater evil.

Will AA's defense prosper?

No, AA's defense will not prosper because obviously there was a conspiracy among BB, CC, and AA, such that the principle that when there is a conspiracy, the act of one is the  act of all, shall govern.

The act of ST, the victim's son, appears to be a legitimate defense of relatives; hence, justified as a defense of his father against the unlawful aggression by BB and CC. ST's act to defend his father's life, cannot be regarded as an evil inasmuch as it is, in the eyes of the law, a lawful act.

What AA did was to stop a lawful defense, not greater evil, to allow BB and CC achieve their criminal objective of stabbing FT.