Can someone explain to me why he can’t kill these people?

Because it would receive backlash from the people and the council has deep roots when it comes to governance so if they were killed immediately without looking for some replacement or without dismantling thier influence first the kingdom's economy would collapse and possibly that the reign of Karas would be rebellion after rebellion from his own people since they were brainwashed into thinking the council always helped them not to mention the council's deep ties to the church.