The foreseeability debate is quite important.
I think the gun analogy does have some relevance.
If you are a technician, you should, through your training and licensing, have an appreciation of why there are approved repairs & procedures. They should also appreciate that engineers will have specified materials and fixing schemes based on analysed requirements and proven testing. You can't change these without approval (e.g. from DA, who may need to confirm there is no impact) and if you do so you should be under no illusions that you should be responsible for the consequences.
In this respect, it was entirely foreseeable that his actions could be detrimental to the airworthiness of the aircraft. For a start the part could have fallen off after the continental aircraft took off, landing in a playground - foreseeable and unacceptable.