The following is my opinion.
My belief when it comes to PKs is that a PK should never be an option for every injury, cut, scrape etc. I believe that a PK should only be an option when:
A: The player has done something stupid OOCly to put that character in a deadly position. For example: Trying to pull a meme and running around an enemy tank, or yelling Leroy Jenkins while rushing a marauder.
B: The person has RP'd that the character is being put into a deadly position. (Some people might just want to RP injury/PK and so will do something intentionally daft in RP)
C: A PK has been requested by that player for that event. ("Sick O' this character" Syndrome.)
D: The situation that the character was in would be generally considered by most as a deadly situation, including, but not limited to: Intense gunfire, fire, missile strikes, vehicle-related deaths (Military vehicles, not Old Farmer Johnson's tractor), falling from a large height and crushing injuries where somebody is/has a limb trapped under a heavy object. (I especially believe this last one is important, as people may not realise it, but getting your leg crushed by a heavy rock can be extremely deadly.)
For example, if somebody gets shot in the foot/hand, they should not have to roll for a PK, considering a foot-wound is something that can be treated easily, and people in World War One were doing it to themselves in order to get out of combat, and the medical knowledge of that time was probably not even a quarter of that of the MI timeline.
Things like dismemberment (Losing a hand/foot/limb) should be a definite roll for a PK, considering the huge amount of blood loss.
Also, things like where somebody decided it would be brilliant to walk out in front of a tank's machine gun, or the turret, or a squad of enemies. A 50.cal+ MG would tear you to shreds, as would a tank shell, as would 3 or 4 enemies, or a sniper shot.
When it comes to staff members playing as event characters whom have the aim of shooting at the M.I, it should generally be considered that they should not be out to cause PKs, more add a "real threat" to the situation, rather than just fighting droves of AI, that even the AI in Rome Total War II would consider to be awful, over and over and over. Things like snipers - Everyone should be aware that a sniper would most likely carry a big gun, that does a lot of damage, and would most likely rip through standard MI body armour or even the helmet. - So in this case, it is not only down to the staff not to go COD Pro MLG and 360 Noscope every player he sees, as aiming a sniper in RP would actually be difficult to get a perfect shot, or would at least take some measure of effort, and to act like a proper sniper, but it is also down to the fact that the characters need to be smart, or risk getting their heads blown off. For example: People who see fit to stand around in the open.
Another thing that irritates me from time to time is how you can take lots of considerably BS damage from various sources (Props moving and hurting you, falling from a height that is bigger than a 60cm ruler, "backwards Morita" and OOCly unintended damage perhaps from somewhere else), and then you take one shot from an enemy and that's it, you gotta role for a PK, whereas every other more fortunate player who hasn't fallen victim to Garry's Mod's glitchy and buggy engine gets to take more hits before they gotta roll.
Most staff members are okay with setting your health to 100 for this, however, there are the few who are either too busy with the event, too busy playing, or just generally don't seem to want to answer the call for whatever reason.
When it comes to rolling to receive an injury, following things should be taken into account:
Body armour.
Type of round/object causing the damage.
The weight of the object.
Is that object blunt/sharp?
How fast that object is travelling
Where it is most likely to hit. (For example, someone leaning out from a wall risks their head, arms and maybe neck/upper chest.
How deserving that situation is of a life-threatening injury. For example, a chair hitting someone in the chest after being thrown by a skinny is unlikely to cause a life-threatening injury.
Last Thoughts:
Another thing, on top of everything else, is that players should be graceful when taking a PK. There have been many times where I have seen people get PK'd for what is seen as a legitimate reason (Such as getting a rocket to the chest, mutiny, etc) and then whine about it to everyone, and then complain when their appeal gets denied.
It is obvious that player characters will be PK'd, pretty much every character will either come close to it or suffer it at some point in SSTwoRP, whether the player wants it or not. It's one thing to have a rant and blow off steam about it, but I strongly suggest that the minority of players whom do complain need to have a better attitude towards PKs. It's alright to get upset about it, as most people will likely put in a lot of effort towards a character, and then to have that all taken away in one fell swoop can hurt, I'm just so glad that on this server the community doesn't retort via the words "It is only a game, don't cry." as that never helps at all. Because as much as it IS a game, it is still something where a lot of effort and time and emotion has gone into that character.
