I was going to say "the vigilante did a better job this time than the last", but then I realised he killed somebody that was going to be killed by the mob anyway. Although he probably didn't know either that the mob would kill the second in line.
And he probably was afraid of killing of an innocent guy again, so no problem.
Anyway, interesting. Darn that Professor.
Who says there was only one original alien? It says "Professor S - Original Alien", not "THE Original Alien", so I'm thinking there can be more than one. It would be pretty hard/depressing for one alien to infect/kill off 15 others. I very much doubt the game would have ended if we had killed the Prof in the first night.
I was under the impression that the aliens could infect one person per night, but as Dylflon says, this probably varies, depending on the progress of the game.