Ghaster was waiting for them to make a move. He knows he should kill them quickly but he wants to toy with them. Power is euphoric. He became able to do some things with lightning that he never thought he could do. To be honest, he stumbled on the revelation but it empowered him the same. That power is his now and it feels great to have. It feels even better when there are others who lack power.
These people who have not yet managed to learn the laws of this world are no match for him. If not for their use of explosives, they would have been hunted like rats with no way to fight back. Instead, they chased him and sent him running like a rat. But the world works in mysterious ways. He is here for payback now. He is not a rat anymore. He is the predator.