Buckingham has been captured and forcefully taken to Salisbury. His requests to speak with Richard have been denied.
Knowing he's been absolutely forsaken by the wicked king, Buckingham notes that today is All Souls' Day, the day set aside to commemorate (and in some cases exonerate) the dead. It's fitting, he thinks, for the souls of all those whose lives have been wrongfully taken by Richard to watch over the king's demise, hopefully taking pleasure in it.
Buckingham begins to take stock of his life: he betrayed King Edward by being unfaithful to Edward's children and Queen Elizabeth's allies, and he realizes he was foolish to trust Richard, who would never trust him (or anyone). He admits he's been a wicked man, and he can see God's justice in having his wickedness be his own downfall.
Finally, like so many others, he thinks on Queen Margaret's curse. She condemned him to have his heart split open with sorrow caused by Richard, and she was mostly right.
Buckingham then tells his guards to lead him "to the block of shame" for his beheading in the marketplace.