In The Walking Dead, "Bury Me Here," things started to go south between the Kingdom and the Saviors, though everything that happened could have been easily prevented. The cycle of violence left two Kingdom citizens dead: Morgan's young protégé Benjamin was killed by the Saviors and Ezekiel's right hand man Richard was killed by Morgan. But why did Morgan kill Richard on The Walking Dead?
It seemed like there were a few layers to the situation, but it was all ultimately related to the tensions brewing between the Saviors and the other communities. Richard was the Kingdom resident who was most open about wanting to fight back and he was willing to do anything to incite Ezekiel to violence. His first plan, relayed to Daryl a few episodes ago, was to frame Carol for the deaths of several Saviors so they would kill her and then Ezekiel would be inspired to attack them.
His second plan, which played out in "Bury Me Here," involved stealing part of the supplies the Kingdom was delivering to the Saviors so the Saviors would punish them – thus giving Ezekiel a reason to fight back. Richard thought he would be the one to die in that situation, but it was Benjamin instead. When Morgan found out Richard was behind everything, grief made him do the unthinkable.
Richard tried to explain himself to Morgan, but Morgan was already spiraling out. He was having flashbacks to his son's death and getting those feelings all tangled up in his grief over Benjamin, who he had recently been connecting with. When it came time to meet with the Saviors again to hand off the missing supplies (in this case, a single cantaloupe), Morgan was at his breaking point. He lashed out at Richard, knocking him to the ground and then slowly strangling him to death – something Ezekiel, the other Kingdom residents, and the Saviors just watched unfold without intervening.
Then and only then did Morgan reveal what Richard had done. At first it seemed unclear whether Morgan and Richard had planned this together, since Richard had been willing to die for the cause and his death did get the Saviors off their backs for the moment. But as Morgan continued ranting, it became clear that he was unraveling. Killing someone reverted him back to the man he was before he was rehabilitated.
Richard's death was sudden and surprising and it looks like it will set Morgan down a darker path.