What would be a word for the part killing the whole?

E.g.: organ failure kills a person. Cancers do this too. Or, figuratively, a person bringing down a group. Like synecdoche and metonym but for murder. Maybe “metacide”?

In a literal sentence: “His heart committed _________ against his body.”

Or, used figuratively: “Aspects of the U.S. are now so poisonous to the body politic that they are on the verge of _________.”

Solution 1:

Consider the verb gangrene.

In its medical sense, it refers to a specific decease, e.g. the decay and feath of body tissues usually in the extremities like the foot and the leg which darken due to lack of blood circulation. Gangrene requires amputation. (source)

In its figurative sense, it means to affect with moral or spiritual corruption and decadence that pervades an individual or group. (source)

Examples:

Bringing to account those responsible for the violations described in this report and making sure that these violations do not repeat themselves in the future will require concerted and sustained action by all of these actors to end the impunity that gangrenes the judicial system. (source)

Our political system is decaying. It’s on the way to gangrene. It’s reaching a critical mass of citizen revolt. (source: Ralph Nader)

Solution 2:

The first sentence might be rewritten like this...

His heart failed his body.

The second sentence could be rewritten like this...

Aspects of the U.S. are now so poisonous to the body politic that they are on the verge of collapse.

But it sounds like you're looking for a single, unifying word. In fact, people who are suffering from defective organs or illnesses are often said to eventually collapse. The lungs, in particular, can collapse, and one might say the respiratory or circulatory system collapsed.

The term Achilles heel also comes to mind. One might say, "The United States collapsed after virtually every major institution had succumbed to corruption, but the nation's Achilles heel turned out to be the education system, which nurtured citizens too ignorant to fight back."

P.S. It's kind of lame, but the term weakest link is also worth considering.