really means ruin, death, or destruction (Anglo-Saxon, bana, a murderer); and “I will be his bane,” means I will ruin or murder him. Bane is, therefore, a mortal injury.
My bane and antidote are both before it. This [sword] in a moment brings me to an end. But this [Plato] assures me I shall never die.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894