I first noticed this in the ultra-boring Ridley Scott epic Kingdom of Heaven, but you can see it in plenty of movies. If there's a recently-dead person who has an important piece of jewelry, like a necklace that is a family heirloom, or even a simple crucifix, the hero will always rip it off their body dramatically, instead of simply unfastening it. The gesture says, "I need to keep this necklace, and also destroy it. My grandfather would want me to have this, and for it to be completely unwearable."
A variation on this convention comes when the hero is knocked out, hospitalized, or otherwise incapacitated, and a villain rips a necklace off of them. That can be seen in the film Walking Tall, when a casino tough tears off The Rock's Special Forces necklace. In these instances, it is sometimes hard to tell why the villain even wants a necklace like that. If it has jewels in it, fine, but why does a non-Special Forces veteran even want a set of dog tags, especially one with a now-broken chain? The answer is, to let the audience know that he will later be killed, definitely by The Rock, and possibly with a two-by-four.
Of course, upon killing his adversary and retrieving the important personal talisman, the hero will again take it off as violently as possible. And the cycle continues.
Also, sometimes the necklace is brutally ripped away, and then moments later is gently fastened about someone's neck. I always waste a lot of time imagining that this is a necklace with a special clasp that gives way when you yank it but doesn't break. Except why would you want such a necklace?
I'm glad you brought all this up, Sean, because honestly this issue has given me a lot of furrowed brows over the years.
Ohhhhh. It's like one of those kitty safety collars. It releases under pressure in case the wearer gets it snagged on a chain-link fence, the neighbor's rhododendrons, or the triumphant yet clumsy grasp of a troglodytic hero. But its clever non-breaking mechanism allows it to be retrieved and reattached for proper identification/romantic scene/proof of ownership.
Given the sexual politics of most mainstream movies I think that last one actually works for both the cat and the generic flowing-haired female lead.