And in the DUST canon, The Courier is higjly related to the cataclysm; after completing the MQ for the NCR or Independent, he was accused falsely by the NCR of some of the atrocities they were doing; waking up the Tunnelers, spray Vault 22 spores, nuking at the end of LR, freeing the Cloud, etc.
Thus, he was hated by everyone and he retired to a cave in the dried out Lake Mead, and eventually became ghoulified and sorta insane.
Apparently, a lot of people and things tried to kill him, and it is possible that he killed Follows-Chalk when he was asking for help.