In S8E16 of the Walking dead, we see near the end that there is a big gun fight happening, but then later in that fight (Hilltop, scavengers, the kingdom vs the Saviours) Rick and Negan are having a deadly fight between each other which results in Rick slashing Megan's throat.
Maggie, Jesus, Daryl, Carol and all the other friends see it happen and they state in happiness...until Rick makes the decision to keep Negan alive.
There is a mention of Rick reciting his son Carl's words in the letter via thought and decides to keep Negan alive...but given Negan is incredibly dangerous and has killed many, even Rick's own and best men...decided to keep Negan alive...
Why did Rick do this? He defeated the governor, so why not kill Negan and end the war entirely?