Because they didn't actually kill the monster and so it followed them to the village after the caravan incident.
The end of the chapter suggested it was when they were chasing mc (away from village) at the end of the chapter, that it started stalking them without them noticing (it was lurking somewhere between village and cemetary).
So those assasins are only supposed to be searching for mc to bring back to the princess unharmed, but now they want to kill him? the fuck is happening?
It's much easier to simply kill than capture somebody alive. Now that he has severely weakened them, they suspect that they lack the strength to capture.
Even though their orders were only find him and bring him to the princess? All she wants to do is thank him but her retard squad wants him dead for no reason.
The assassins are the rival kingdom's goons. The ones that tried to assassinate princess and failed due to him. They are tracking him down to either convince him to join them, kidnap, or kill, with preference being that order.
With them judging it impossible to kidnap someone so powerful, they decided on the "assassinate when he is unawares" course of action.
I, too, don't get why they don't want to even ask him if he wants to join their kingdom, but I suspect it is because they doubt the country can keep him locked up if he does, and they think that that killing him becomes a non-option if he knows there are assassins around (so better go for the "assured" option).
But either way, they are not the princess's goons, the adventurer party they infiltrated is.