Hamish's story starts at janitorial school, where he was ridiculed for his low space diaper cleanup performance. After gradutioan Hamish became determined to take over the entire janitorial industry to prove himself.
Furthermore, the record shows that the Covenant war was actually a campaign to find and capture Hamish Beamish after they learned that he stole sensitive Covenant/Foreunner material regarding advanced janitorial technology that would forever change the galaxy and redefine his role as a janitor.
No one can explain how Hamish obtained such technology, but it became clear that he planned on using the technology for revenge against those who tormented him. The technology was so efficient that Hamish's influence and power would have affected the various Human colonies, causing all the janitorial agencies to go out of business.
Many attribute the Covenant's discovery of Reach and Earth to Hamish's colony hopping. "Wherever Hamish Beamish goes, you can be sure that the Covenant will follow", a common saying amongst humans.
Feeling responsible for the millions of human deaths, Hamish Beamish went into hiding.
The space mop scene makes sense. He knows that he has compromised everyones life and now has to live knowing about the impending tragedy.