What's your definition of what it takes to be labeled a true monster?

The Joker

Monsters don’t plan out crimes or assess the damage they might cause. They just walk thru and knock down buildings or stomp people, or destroy whatever is in their way.
To this definition, I nominate Richard Nixon.

I would consider a monster a person that has lost their sense of humanity. A person with no sense of respect or appreciation for anyone else in the human race. People like Ed Gein, John Wayne Gacy, Jack the Ripper, Jeffrey Dahmer, insert more generic serial killers here.

My ex and my MIL! “They’re everywhere!! They’re everywhere!!!”

Michael Woodmansee
And those mentioned by @KatetheGreat

Someone or thing who is evil, knows they are evil, acts on their evil intentions and doesn’t care that they are evil or have any qualms about repercussions. Bonus evil points if they take special pleasure in it.

I think many Politicians fit the bill.
Those who wish to give tax cuts to the wealthy, take away from the disabled, elderly and Middle Class.
I think cuts should begin with Congress. The deficit could be cut by ending their free Cadillac Insurance.
They are my defination of living breathing immoral Monsters.

A extendable mouth inside another mouth with 5 rows of razor-sharp circular teeth, and 16” claws on each of 8 arms, and a voracious appetite for human flesh (especially that of young pretty girls).

There is only one name that springs to mind.
Sharktopus.

What @KatetheGreat said. A real monster is someone who lacks anything that makes them human.

A true monster = a prohibitionist.

A monster? Well. A Dalek is a monster. A being whose only goal for its existence is to exterminate other creatures, no matter what? Monster.

I just realized my answer could also be applied to my office manager.