Simple snippet on how to delete all users of a given role in drupal 8.
use \Drupal\user\Entity\User;
$role = 'editor';
$ids = \Drupal::entityQuery('user')
->condition('status', 1)
->condition('roles', $role)
->execute();
$users = User::loadMultiple($ids);
foreach($users as $user) {
$user->delete();
}
