Rodrigo has taken the guilt on himself, and knows the shot is coming; the aria is his farewell to Carlos, a friendship set to a melody so even that the violence around it barely disturbs the line. Verdi wrote Don Carlos for Paris in 1867 and went on revising it for the rest of his life — five acts, then four, French, then Italian — but this page survived every version. It is among the reasons the role is coveted by baritones who could be singing villains instead.