Killing Firebead Elvenhair at the beginning imbalances the game

I just noticed that after killing Firebead and leaving Candlekeep all NPC:s were above their normal starting level. For example Imoen was lvl 4 and Montaron/Xzar also lvl 3-4. Then i went to kill all the monsters in that first area and it was super-easy.

Does this occur only in the enhanced edition or vanilla game also?
Should he just be left alive because of this?