In Betrayal, does a monster really die after 1 hit?
Looking at page 18 of the Rulebook there's a section of How Monsters Work which says:

Monsters can't be killed.. If a monster would take any damage, it is stunned and misses its next turn... Stunned monsters can't slow an explorer's movement. Even if a haunt says to do something other than stun monsters when they take damage, they can still be stunned by effects that specifically stun monsters.

These are the base rules for monsters which can be manipulated by specific haunt rules. Do note that the Traitor may know game altering rules that you, as a Hero, simply do not know.
Now, let's take a look specifically at Haunt 46:
Below are minor spoilers for Haunt 46 including special rules. It is brief and only outlines the rules in question. Read at your own risk!

 The Feast!
 Cannibal Freaks ( Speed 2, Might 4, Sanity 4 )Special Rules:
 If a Cannibal Freak is defeated by an attack, it's killed instead of stunned.
 In this scenario the special rules dictate that instead of getting stunned the monster would be defeated instead. That is if they take any mental or physical damage. That being said, specific items that stun can still be used to incapacitate the monster. The monsters will die in one hit but the base stats for the monsters are also fairly high. Depending on how many items the Heros were able to attain during the game you may need to play this safe and cluster your monsters together or corner a specific hero to get them out of the game.

Do note not all Haunts are perfectly balanced and some rules may not be flushed out enough to answer every single question that may pop up. From Avalon Hill:

Keep in mind if the rules are not specified in the rulebook you and your group can decide what makes the game most fun.

In this case though, I think the answer is cut and dry.