The Emojibots as well as the Vardy were made to keep all of the colonists happy at all times. When a woman died of natural causes, her family and friends began to grieve. The Vardy read grief as an enemy of happiness and as a result killed anyone experiencing it. Eventually, all of the colonists who were awake at the time were killed, their skeletons used as fertiliser.
The Twelfth Doctor and Bill were handed mood badges so that it was able to keep track of what their emotional status was. It was able to read when Bill was hungry and fed her jelly. When they found out that the colonists were murdered, the Emojibots read that they were upset so began chasing them and surrounding them in order to kill them. The Doctor found out that by smiling and becoming happy the Emojibots would let you go.
When the remaining colonists woke up, the Doctor explained that the few colonists awake before were murdered, but before listening to him completely they took up arms in order to fight the Emojibots. Praiseworthy had stumbled across the robots but expressed grief for his mother, causing the Emojibots to grip him in order to murder him. Steadfast shot one of the two, causing an angry response to which it summoned Vardy to kill the colonists. The Doctor realised that the Emojibots were a lifeform, so wiped its memories of the events that had occurred. This meant that the Emojibots thought Gliese 581 D belonged to them. The Doctor started a negotiation between the humans and the Emojibots in order to try and allow the humans to live with them.