EDIT - Simple misplacement of Space.World was killing my brain. Read below reply to see how to use simple camera rotation left and right on Y if you are having the same problem as me.
using UnityEngine;
using System.Collections;
public class MouseRotation : MonoBehaviour {
private float mouseX;
void LateUpdate(){
HandleMouseRotation();
mouseX = Input.mousePosition.x;
}
public void HandleMouseRotation()
{
var easeFactor = 10f;
if (Input.GetMouseButton(1))
{
//Horizontal rotation
if(Input.mousePosition.x != mouseX)
{
var cameraRotationY = (Input.mousePosition.x - mouseX) * easeFactor * Time.deltaTime;
this.transform.Rotate (0,cameraRotationY,0);
}
}
}
}