When a monster that is equipped with a union monster like Spirit of the Six Samurai changes control from a card's effect, and that monster manages to destroy another monster as a result of battle, the new owner of the monster does NOT gain the effect of the equipped monster (Spirit of the Six Samurai) in this case.
The reason is the same as why you cannot prevent an opponent monster's attack by equipping Safe Zone to it: You are NOT your opponent.
The reason is the same as why you cannot prevent an opponent monster's attack by equipping Safe Zone to it: You are NOT your opponent.