@Kaarme The kind of complex plot they're running is the kind where the less people know about it the better, they've gone to great lengths to cover their tracks and everyone else involved besides the woman that faked her death has been killed off already, they could have easily involved the doctor that handled the bodies in the plot but instead they had the woman seduce him and then trick him, most likely because they would have needed to kill him like the others after she escaped, and that would have raised questions when added to body count that already exists.
It's the same thing with that guard, going by the post they gave him (protecting a medium ritual) which is something they take very seriously as multiple aristocrats are involved, he's someone that people would absolutely investigate should something happen to him, and even before that, if Jinshi (who is very likely the son of the emperor) died while performing the ceremony and the guards outside allowed it to happen they could very well be put to death for failing at their job. At the very least they would be punished and demoted. There is no reason to believe they would want that to happen, their military careers would be down the toilet and their reputation (face) would be completely gone. Also the possibility of their entire families being wiped out exists as well if anyone caught wind of them being involved in the plot to kill someone as important as Jinshi.
There aren't any benefits that outweigh the risk of that, because they would get in trouble no matter what if someone died during a ceremony they were tasked to guard.