Destroy happens immediately

I'm currently working on a Starfox-ish game. I've managed to get quite far today, as I've managed to make an enemy fly towards the player. What I want to happen is that once an enemy flies within a certain range of the player, it will fly over the player and then self destruct. This is a behaviour I've called "disengage".

At first I was uncertain how to make the enemy fly over the player (because midair collissions with every enemy would be stupid), then I created an actual prefab which is a child to the players plane, which the enemies try to fly in to.

However, once I wrote the code, I wasn't certain, but I thought it was targeting the "disengage target" straight away. Once I added the self destrct, however, I realised that the enemy plane self destructs immediately. It seems to bypass the timer, as it gets destroyed on frame 1, even if I set the timer to ludicrously large numbers, and the disengage distance to ludicrously small distances.

Anyways, here's the code:

 using UnityEngine;
 using System.Collections;
 
 
 public class EnemyFighterScript : MonoBehaviour {
     public float interceptvelocity = 10.0f ;
     public Transform Player;
     public float distance;
     public Transform target;
     public Transform disengagetarget;
     public float disengagedistance;
     public bool disengage;
     public float timer;
     // Use this for initialization
     void Start () {
         disengage = false;
     }
     
     // Update is called once per frame
     void Update () {
         distance =Vector3.Distance(transform.position,target.position);
         if (distance<disengagedistance){
             disengage=true;
         }
         if (disengage=false){
         transform.LookAt (Player);
         transform.position += transform.forward * interceptvelocity * Time.deltaTime;
         }
         if (disengage=true) {
             //makes enemy fly away and prepare self destruct after reaching proximity to player
         transform.LookAt (disengagetarget);
         transform.position += transform.forward * interceptvelocity * Time.deltaTime;
         timer =- Time.deltaTime;
 
         }
         if(timer <= 1){
             //destroys NPC once it passes player
             Destroy(gameObject);
         }
     }
 }