The denial of convention
because Loki should have
been able to do it
Baldr is at Loki's mercy, yes, but this is no cause for worry. He knows how things will go. Loki already has Heimdall's blood on his hands, spilt at Baldr's request and not Odin's. All one needs to do is remind Loki of this, in Heimdall's voice, and Loki will be unable to do anything but listen, and regret, and offer the unwitting benevolence of inaction. It is convention, after all: in the crucial moment, the one with the upper hand always falters. Loki's capacity for guilt is slight, but still enough for him to show some measure of mercy. That brief lapse is all that Baldr needs. He thinks he has found it now, for Loki is no longer looking directly at him; in that unfocused gaze Baldr sees only undisguised sadness, which is the same as weakness. His smile widens as he readies himself to strike the staff from Loki's hand.
But Baldr never manages to do so, for when Loki moves it is fast and sudden and with none of his earlier reluctance -- his fingers curl, jerk away, the eye disintegrating into dust as it is torn free, and Baldr's shocked cry turns swiftly into a scream of pain.
Heimdall's blood has drawn a thin line across Loki's cheek. Loki does not wipe it away until he is sure that the ragged sound of breathing is Heimdall's and Heimdall's alone.