protagonists like that are popular because a large number of people have no concept of danger or understanding of survival, people in europe have been put in jail for defending themselves, people go to dangerous countries assuming everyone is peaceful and get killed. even the incredibly basic concept of killing to eat is beyond a lot of people in 1st world cities.
this is a cliche chinese fantasy story, the world is black and white, bad guys are evil, people only respect strength.
the alchemist guy started problems on their territory and tried to push them around, he is an active antagonist not just an innocent guy doing his job. killing everyone is also the opposite of short-sighted.