mmmm... water again
Added 22 minutes later
Let's assume Naira is a serial killer (water, cat meow). Then it's really beneficial for the Mafia to kill her - she swings the axe randomly and might hit them! But how could they have figured that out? I would have 100% killed Stan if I were a Mafia member, because I was sure. Although they have more information: they know each other. So far, everything.
The Mafia wouldn't kill Stan even if he were a serial killer, because the Commissioner would likely check him out anyway. And even if they knew, as long as there are more townspeople than Mafia members, it's beneficial for them to have him swing the axe, because the probability of him killing a townsperson is higher. (Although, of course, for a serial killer, being alone with >=2 Mafia members is a guaranteed death sentence, but being with townspeople is a completely different story).