The explicit meaning in this passage is best expressed by the alternative B: "Brutus told you (<em>Brutus hath told you </em>) Caesar was ambitious. If it's true that Caesar was (<em>if it were so</em>), then it wasn't a good thing (<em>it was a grievous fault </em>), and he's been punished for it (<em>and grievously hath Caesar answer'd it </em>)."
Grievous: <em>having very serious effects or causing great pain.</em>
