Why does the machine ambassador destroy the human leadership after accepting their surrender?

In the Animatrix, the human leadership surrenders to the Machines in the UN building in New York City. The machine ambassador speaks on the terms of the surrender and then "signs" the document to accept it while the human leadership looks on.

This is the scene: Hand Over Your Flesh

After the surrender is accepted, the machine ambassador detonates a hidden nuclear device which destroys much of New York. The human leadership that just surrendered is obviously killed in the blast.

My question is: why did the Machines detonate the nuclear device and kill the human leaders, along with much of New York? What purpose did it serve?

From what we've seen in the rest of the Matrix, the Machines are cold and calculating but not butchers. When there is sufficient cause for violence, the Machines will use it to accomplish their goals, but what goal called for them to destroy the human leadership?

I realize that the instrument of surrender allowed the Machines to take essentially any action without reneging on the terms, but the detonation of the nuclear device seems irrational. For example, at the end of Revolutions, the Architect agrees to free the humans that wish to leave the Matrix. The Oracle inquires if he means to keep that agreement and the Architect seemingly scoffs at the idea of reneging. That suggests that the Machines would not make an agreement they did not mean to uphold, and it seems logical that the Machines would not take any extraneous action beyond the terms of the agreement (e.g. actions out of spite, revenge, etc.).

That is, once the Machines accepted the surrender of the humans, there was no need to kill the human leadership. The humans had been subjugated, the leadership usurped, and the Machines were given complete control. What reason did they have to detonate the nuclear device in the UN building?