That is a step on which I must fall down, or else o'erleap
Antithesis - "fall down" and "o'erleap" highlighting the two options
"o'erleap" - conveys a desire to overcome obstacles swiftly and aggressively
"Fall down" - indicates caution
Modal verb - "must" inevitability of Macbeth's decision
Metaphor - "Step" for an obstacle or barrier in Macbeth's path