There has been some talk on this recently, given the current contemporary operating environment.
It comes down to something like this:
A weapon is a tool. Tools can be used offensively or defensively, or to prop open a door or as shovel.
Canadian personnel subject to the Geneva Convention, regardless of the environment or adversarial compliance with the Geneva Convention will only use weapons defensively to protect themselves or their patients. Can a aggressive fighting defence exist? Most certainly... if required based on the threat. We have a number of examples from the current conflict.
So then we look at the concept of weapons for CFHS personnel, based on the War Establishment. These include the service pistol, the service rifle / carbine (although there are no C8's on the TO&E), the light machine gun for area defence (Role 2 and 3), and the Claymore Command Detonated Weapon (Role 2 and 3). If it is on the TO&E for the War Establishment is it save to assume that one can use it in a defensive role. Most notably missing is the fragmentation grenade, which has been deemed but someone to be a weapon designed more for offensive purposes than defensive purposes and hence not on the TO&E for HS units.
The C6 on armoured ambulances, is still a fiercely debated topic and is a frequent ethical topic debated at all officer ranks and within the JAG (I am not sure I have ever heard and NCM debate the concept much). It is a current exemption to the rule, based on a sound legal opinion and requires a semi-annual review and sign off by the Generals involved in assuming the liability and risk for allowing such an activity. We will see where this settles when all is said and done at the end of Afghanistan operations. The War Establishment TO&E does not have this weapons system on allocation.
You are in a position that is being attacked and a collective defence needs to be mounted with you as the #2 on the mortar... is this ok? I would think so, it is given that a collective defence is required to assure your survival and you are just doing your bit.
You are trying to prep fire a target with the the same mortar prior to it being assaulted with troops as part of an offensive operations with you as #2 on the tube... is this ok? I would suggest no.
You need to fire illumination... I would suggest this is fine, regardless of the offensive / defensive situation as the weapon system is not being used directly for killing but rather as a flashlight.
In the end we all need to live with our actions at the end of the day and we are all grown ups.
There are some other issues with respect to the GC and war that are a little more complex and that I will not get into here.
Just some food for thought based on the current state of business.