Мафия. Learning to Live

Where did you get such accurate information, if you don't mind me asking?

Methods of assassination.

Added 8 minutes later

Unfortunately, anyone could have killed Arandor - for obvious reasons.

If the motivation is simply to kill him for who he is, and not to plot intrigues, then it could have been anyone except:
Whirl, Stan, Hermit, and Cash - for obvious reasons.