I've changed my mind, let's vote out Heroist! Quickly!
I'm a little worried about my fate tonight, and I need to go home now, so I'll say what I have to say now.
If Arandor is a Russell guard, then Heroist is almost certainly one too, specifically Markelius. There are three significant reasons for this: the recent dialogue (Ara throws her support behind Heroist to clear his name, since Ara will be executed anyway), and Ara's response to my crazy post about Heroist. The criticism there is certainly justified, but I find it strange that this criticism even appeared in response to that craziness, and the criticism was so well-argued and justified that it seems like an attempt to divert attention.
If Arandor is from the Cartel, then Heroist could also be from there, but the chance is around 60/40 – i.e., less than in the first case. We need to think about this.
If Arandor is actually innocent, then everything described above is incorrect.
tl;dr:
If Arandor is a Russell, then Heroist is 90% likely to be Markelius.
If Arandor is from the Cartel, then Heroist should be looked at to see if he belongs there as well (most likely Damien).
If Arandor is innocent or Markelius, then ignore this post.