What incident late in Act 5 of Macbeth best echoes Malcolm's "Nothing in his life / Became him like the leaving of it"?
Malcolm, of course, is talking about the Thane of Cawdor who went to his death nobly in Act One. Though, were his lines in Act 5, he might well have been talking about Young Siward, who bravely challenges and fights Macbeth in Act 5, Scene 7:
Thou liest, abhorred tyrant; with my sword
I'll prove the lie thou speak'st.
Siward is slain by Macbeth, and lies dead on the battlefield. Ross tells Siward (Young Siward's father), after Macbeth's death, that Young Siward has been killed:
Your son, my lord, has paid a soldier's debt:
He only lived but till he was a man
The which no sooner had his prowess confirm'd
In the unshrinking station where he fought,
But like a man he died.
Young Siward only lived to be a man by seconds: his "unshrinking" bravery made him a man ("his prowess confirm'd") and then he was immediately killed. Nothing in his life, indeed, became him like the leaving it. And that, indeed, is more or less what Siward says:
Had I as many sons as I have hairs
I would not wish them to a fairer death.
And so his knell is knoll'd.
Well, in Act V is the event of the entire play where Macduff and Macbeth finally meet in battle. After Macduff informs Macbeth that he was born by Caesarian section and not "born of woman" as Macbeth interprets the witches' prophecy, Macbeth knows he is doomed, but fights Macduff anyway. Macbeth is defeated, and Macduff exits carrying Macbeth's body with him to the other soldiers where all can see that the tyrant is finally dead and Scotland can begin her recovery.
I would say that this event definitely applies to the quote Malcolm gave in Act I, as well as the speech he gives before Act V is ended and Malcolm's reign of peace and prosperity in Scotland begins.