Barnavelt continues talking until the very end. As planned, the executioner lops off his head while he is in the middle of a prayer: "I come, I come: o gracious heaven: now: now: / Now I present—[head struck off]" (5.3.182-3).
Barnavelt keeps speaking until the very end, as part of the planned execution. The executioner beheads him while he is in the middle of a prayer, with the words 'I come, I come: o gracious heaven: now: now: / Now I present—[head struck off]' (5.3.182-3) still echoing.