Is this justifiable homicide?

If you're carrying around a 4 lb steel wrench, and they're not carrying any weapons, then you can be charged with murder if someone dies. Even if you have a job at a mechanics shop or somewhere that justified using a wrench that size, you would be asked why you didn't leave it at your job? The preponderance of evidence would be against you, and it would be concluded that you committed premeditated murder. I would advise you to walk another route. If that's not possible, then you need to tell someone (not just your buddy) that you have been threatened. That way, if something does happen, at least there will be evidence that they were planning an attack on you. Even though you have the wrench, if there are eight of them and only one of you, I'm afraid you will be in for a bad time. If they took the wrench away from you and beat you with it, it would be too bad.