Supreme Leader Snoke was a master of the dark side of the force. He used his abilities to gather the remnants of the Empire and restructure it as the First Order.  Even though he wasn’t a Sith he followed many of their tenets. And, like Palpatine before him, Snoke saw the Jedi as a threat to his consolidation of power. If he could destroy Luke Skywalker and his Jedi padawans then no one would be left to stand in his way.

One of those padawans was Ben Solo, Grandson of Darth Vader. Snoke saw his dark side potential and managed to lure him away from the light.  Ben took the name Kylo Ren and with a few of the other students formed the Knights of Ren. They killed the rest of the padawans leaving Skywalker as the last Jedi.  

Snoke thought his position was secure until he became aware of Rey, another potential Jedi. She and Kylo Ren shared a mental connection through the force. Rey thought she could Redeem Kylo and confronted him. The connection was actually part of Snoke’s plan to capture Rey. But instead of helping to lure Rey to the dark side, Kylo betrayed and killed Snoke and took his place as Supreme Leader of the First Order.