Problem with my firing script

I have a problem with my bullet once it fires. I have a script that checks for a bool if the player is facing left or not. And what I want to happen is that when the player faces the left direction and fires the bullet will go in the direction that he is facing. The script below is attached to the actual bullet and the bullet itself is the gameobject that checks for the bool. And it will work fine until I turn left the bullet would start to shake uncontrollably. What am I doing wrong here?

public float speed;

	public float death;

	// Use this for initialization
	void Start () {
	// Update is called once per frame
	void Update () 
			speed = 1 * -speed;
		transform.Translate(0, 0, Time.deltaTime * speed);

	IEnumerator DestroyBullet()
		yield return new WaitForSeconds(death);

The problem is this: when the player is facing left, you’re changing the sign of speed each frame (positive to negative then back to positive etc).

Instead of modifying speed directly, you could declare a new variable inside Update() (for instance, “signedSpeed”). Set signedSpeed to equal speed, and negate it if the player is facing left. You can then pass signedSpeed in to your call to Translate().