You will not discover who killed George until some time after leaving Aleroth. When you get to Verdistis (to the far east and north) you will hear about some more murders...

If you are past that point;
In the cellar of one of the shops, there is a partly hidden room, where you will find George's vampire drug dealer from the start of the game. Make sure you go through all the speech options with him before turning hostile, so you can learn enough to report back to Mardaneus and solve the quest of George's murder.


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