The Master of the Kanima is the being who controls and uses the shapeshifter to carry out the Master's vengeance.
The Master will use the shapeshifter to do his bidding, carry out his orders unquestioningly and kill those the Master deems so.
Both antagonists controlled Jackson to commit their crimes throughout Season 2.
The Kanima's Master shares a psychic bond with the shapeshifter. Both of them form, implement and solidify their supernatural bond by pressing the palms of their hands together.
The Master can communicate with the shapeshifter telepathically, he can sense his servant from miles away, as well as speak through the Kanima by a use of mental projection.
The supernatural bond between the master and servant causes the phobias of the Master to be shared with the Kanima. In his kanima shape, Jackson shared Matt's fear of water, though in his human guise he's a natural swimmer. The Master's personality will also affect the bond influencing the kanima servant's behaviour.
In Raving, Matt spoke through Jackson referring to him and Jackson: "We're here."
In Season 2, when Jackson became a kanima, his first Master came to be Matt Daehler. Matt was an unhinged teenage boy who held a vendetta against a group of people involved in his near-drowning years earlier when he was a little boy, including Mr. Lahey.
Matt, at one point, unintentionally took a photo of Lahey. He looked down the lense of his camera, feeling an unbelievable rage, wishing Lahey was dead. Jackson, who was newly transformed and thus seeking a Master, reacted to this vengeful feeling elsewhere and carried out Matt's vengence. When Matt first saw Jackson in his kanima shape, the shapeshifter was instinctively attracted to him, and vice versa. Once bonded to Jackson, Matt saw that the Kanima had killed Lahey for him.
Matt figured out that Jackson would kill again for him. He visited Tucker Cornish, another individual involved in the incident and took another photo. Matt realized the bond between him and his servant had bestowed these new traits power upon him. He just had to take a photo of his victim, or think about it, then Jackson would take their life. Matt used his servant to kill all the people involved in the incident.
In Fury, Matt Daeher is killed by Gerard Argent. Gerard then becomes Jackson's new master by greeting the Kanima afterward. This is most likely due to Gerard being a psychopath and his deceptive character, qualities the Kanima presumably searches for in its Master. This could explain why Jackson didn't attack Gerard in Frenemy, because the shapeshifter saw Gerard as a noteworthy Master.
If the Master uses the shapeshifter to kill innocents, or if the Master takes lives himself, that breaks the rules of the curse. Matt committed such acts and he began to consequently transform into a Kanima himself.
Derek Hale: You can't just break the rules. [...] The universe balances things out. Always does.