Smoothly Rotate the Object

Hi there. I'm trying to rotate my camera slowly. The first attempt is succesful. (from 0,0,0 to 0,90,0) but the others just failure.

Can you help me?

 void FixedUpdate()
     {
         if(Input.GetMouseButtonDown(0)) 
             {        
                 Ray ray = Camera.main.ScreenPointToRay(Input.mousePosition);
                 RaycastHit hit;
                 if(Physics.Raycast(ray, out hit)) 
                 {                
                     if(hit.transform.tag == "SagTus")
                     {
                         angle += 90;
                         Debug.Log(angle.ToString());
                         sag=true;
                     }        
                 }
                 
                 if(Physics.Raycast(ray, out hit)) 
                 {                
                     if(hit.transform.tag == "SolTus")
                     {
                         angle -= 90;
                         this.transform.rotation = Quaternion.Lerp(Quaternion.Euler(new Vector3(0,0,0)),Quaternion.Euler(new Vector3(0,angle,0)),Time.time);
                     }        
                 }
             }//end input if
             
             if(sag) 
             {
                 this.transform.rotation = Quaternion.Slerp(Quaternion.Euler(new Vector3(0,angle-90f,0)),Quaternion.Euler(new Vector3(0,angle,0)),Time.time * speed);
                 
                 if(this.transform.rotation == Quaternion.Euler(new Vector3(0,angle,0)))
                 {
                     sag = false;
                         
                 }
             }    
     }