People who saw the thief ran after him.
A transitional modifier would not necessarily require a comma of any kind if it is at the very end of a sentence, although this is very much up to the will of the author.
This quote means that whoever wrote this quote thinks that people who are dead (which is what he means by inhabitants of the deep) are a lot more happy than him/her.
Because it leaves almost no trace and no symptoms yet can still kill you very quickly and effectively
