I'm trying to make a first person camera for my game but..

I've made a first person view for it and the camera rotates and everything there's just one problem, if i move it enough the camera rotates A little bit of of sink and its really annoying

Note:It starts out straight. Here's my code

 using UnityEngine;
 using System.Collections;
 
 public class CameraAngleF : MonoBehaviour {
 
     public float mx = 0;
     
     // Update is called once per frame
     void Update () {
         float cx = mx * Input.GetAxis ("Mouse X");
         float cy = mx * Input.GetAxis ("Mouse Y");
 
         if (Input.GetKey (KeyCode.Mouse1)) {
             transform.Rotate (0.0f, cx, 0.0f);
             transform.Rotate (cy, 0.0f, 0.0f);
         }
       
     }
 }
 

i want it so the it go's straight like this : not diagonal.