Bloodline Curse, Old Hunter. What's the effect of killing the skill upgrade NPC?

In " Blood Curse ", the NPCs who leveled up their skills were usually some special monsters. They had special skills and attribute upgrades, which could help the players improve their combat ability. If one killed an NPC who leveled up their skills, they might lose these special skills and attributes, and might also lose experience points and gold coins as rewards. In addition, if you are attacked by NPCs, you may receive additional damage. Therefore, if one wanted to level up their skills and obtain better rewards, it was best not to kill the NPCs that leveled up their skills but to wait for them to die before leveling up. At the same time, they had to pay attention to avoid being attacked by NPCs to avoid receiving additional damage.