Killer

One who or that which kills.

"Carbon monoxide is a silent killer."

That which causes stress or is extremely difficult, especially that which may cause failure at a task.

"That test was a killer."

Something that is so far ahead of its competition that it effectively kills off that competition.

"Digital streaming platforms are CD killers."

A knockout form of darts or pool involving several players.

Excellent, very good, cool.

"killer feature"

Causing death, destruction, or obliteration.

"killer bee"

Distressing, uncomfortable.

"I had a killer headache this morning."

A sea mammal related to dolphins and porpoises (Orcinus orca).