Goblins and goblets
Yes, while monsters can include both the axe murderers and the supernatural, by convention the term is mainly used for those big weird looking creatures that don't fit any other description. You won't find many ghouls in a book called Where Monsters Dwell.

Even with gi-normous creatures, there's a question of association. Ogres and sea serpents fit the classic mold well, but we think of them as creatures of myth and legend, and sea serpents have the same problem of adaptation we saw when looking at adventure - they thrive on mystique, in a narrative build up containing the unknown and the unseen; which is dashed when you have to draw the thing.

Let's consider poisoners:

• traditionally they sit within that genre so reviled by our friend, Dr Wertham, the crime comic. That is if we see them in a fairly straight setting slipping something into their victim's drink - say, to get at the inheritance.
  
• if the cops apprehend this fiend, it's still crime, though if the police take centre stage as protagonists, it will likely be 'detective', a subset of the crime genre
  
• obviously if the poisoner holes up in a haunted house, it's horror, but the genre also seems to incorporate those tales where the cad comes to an untimely end through divine (or diabolic) providence
  
• if the poisoner wears a bright costume and calls himself Toxin it's superhero
  
• if the poisoner is laying them out in the trenches it's war

All clear?