So I haven't read this anywhere before but I have a theory about the bosses and other higher health pool AI. To put it simply most of us know that Terragroup the pharmaceutical, bioengineering, giant has been hard at work on human experimentation in the Labs below Tarkov based on the testing rooms with hospital beds set up behind protective glass
So Killa is not just a Joke mall cop he is there to protect TG interests in and below the mall. It's said by Jager I think that he was a Athlete in his former life, this is how he came into contact with TG and they preformed whatever process on him to improve his Athletic ability. After that I think he was in Tarkov to continue treatment and be monitored by TG. I'm uncertain as to the reason for his willingness to defend the Ultra Mall hence why he may be puppeteered (think Manchurian candidate but with Captain America) by TG in the same process that makes him now superhuman in terms of damage he can absorb and how quick his reflexes are. More evidence comes from his Stash container is a TG marked shipping container where he Stores his weapons and then the safe room itself is where I think Killa lives when he's not murdering anything and everything in the mall. Add that the James bond urinal to swipe SR11 which is a TG keycard and the secondary WR22(?) keycard container in the storage yard. Though what I think he is protecting is deeper underground of the Ultra mall note that below the Escalators that lead from parking garage to Goshan the lower parts of the lower garage is flooded and I don't think its a accident I think its to keep something hidden or inaccessible, thats what I think Killa is truly protecting.
Glukhar is a former Russian Marine likely some form of special forces as well as his followers. I think they were approached by TG or perhaps TG was working with the RU govt on all this hence why they doing this in Tarkov at all and the whole conflict is over TG not wanting to uphold their part of the deal to the RU government. Digressing they are at reserve along with the Raiders at the behest of TG to protect something secret in the bunkers below reserve or to find something. Once again part of the bunker area is flooded in the same way the lower garages are at interchange, though this time I think it was more to keep TG out as the base was obviously held by the RUGF. Another tangent theory is that TG has picked up where a old Soviet super solider program left off and put their modern tech, knowledge, funding and focus on figuring it out, hence their interest in the bunkers.
Reshala (The Deal Maker) I think is either a super soldier plant placed at the factory by TG to keep a eye over TGs interests at the factory (manufacture of Blue ICE) or knew of the mystery substance and or others being worked on at the factory and has used it for himself and his muscle. Though in the gunsmith quest to mod a MPX the Mechanic remarks that Reshala and his goons whom your making the weapon for someone to kill them are former military as well. Simply put I think Reshala is at customs defending the Factory and making some money on the side hence the name "The Deal Maker" dealing in what IDK.
Shturman I have not a fucking clue he seems to be a dickhead TG super solider that is supposed to be defending something at the sawmill though IDK wtf it could be.
Sanitar (Orderly), to me he seems to be a opportunistic orderly that had access to the knowledge and process required to make super soldiers and used it on himself for his own gains after the collapse in Tarkov. Pulled from the Tarkov wiki " A former doctor, he worked in the health resort "Lazurny bereg", and before that in the TerraGroup laboratory. After the events that happened in Tarkov, he gathered a gang with former colleagues and operates on the "Shoreline". Actively uses professional skills in combat, quickly healing himself and the gang members. "
Also last closing Spaghetti thrown at the wall. Jager… his missions are shit literally because he is trying to get you killed and hoping you take out some other assholes in the process. Think about who he likes, Therapist who is trying to help make the situation better for innocent people in Tarkov and Mechanic who is mostly neutral to the goings on around him. Now who doesn't he like, Skier who seem almost like a Mob man who only cares about himself and money, Peacekeeper who out of everyone's quest actually wants the highest amount of people killed in all quest lines and actively contributes to the worsening situation in Tarkov, and Prapor who in lore is aligned with BEAR PMCs of which BEAR PMCs have been shooting up a whole lot of Tarkov which Jager doesn't like. Which leads to you the player a PMC whether BEAR or USEC Jager doesn't like you because your part of the problem destroying Tarkov, his home, so he uses your greed and ruthlessness to get ahead against you and gives you terrible quest in hopes that you get yourself killed in the process and if not at least you killed so other bad people along the way as well.
TLDR: TERRAGROUP BAD MAKE BOSS SUPER SOLDIER POSSIBLE MACHURIAN CANIDATES, AND JAGER TRYS TO KILL YOU ON PURPOSE NO JOKE. SORRY FOR WALL OF POORLY WRITTEN TIRED TEXT.