My reasoning for showing that they are a monster right away:
You're going to need tension in your story. Now, that tension can come out in other ways if you go with the 'twist' version of your story, but by revealing they are a monster right away, you have a lot more room to play.
If the reader knows they are a monster, you have the opportunity to have greater character development. Readers have the opportunity to sympathize with the character as they go through their dramatic highs and lows (moments in which they display humanity or monstrosity), and are able to understand how far the character has come.
As well, for tension, by knowing the character is a monster right away, you have the choice to raise tension by having points where the character is trying (and nearly failing) to hide their true nature. The reader can understand these moments, knowing what is at stake. Not only that, but for moments of internal character tension, the readers can understand the real stakes of the character's troubles.