Word for someone or something that always ends on a negative note

An AmE slang expression for the circumstances OP describes might be a “downer.”

downer noun: slang 1. A depressant or sedative drug, such as a barbiturate or tranquilizer; 2. One that depresses, such as an experience or person. See, The Free Dictionary

The epitome of a “downer" personified, is “Debbie Downer”.

Debbie Downer is a name of a fictional Saturday Night Live character who debuted in 2004, and who was portrayed by Rachel Dratch.

The character's name, Debbie Downer, is a slang phrase which refers to someone who frequently adds bad news and negative feelings to a gathering, thus bringing down the mood of everyone around them. Dratch's character would usually appear at social gatherings and interrupt the conversation to voice negative opinions and pronouncements. See, Wikipedia

Debbie Downer at Yahoo!

The closest thing I can think of is buzzkill:

noun

North American informal A person or thing that has a depressing or dispiriting effect:

From Oxford Dictionaries Online

Or perhaps spoiler:

noun

1 A person or thing that spoils something:

From Oxford Dictionaries Online

or pot stirrer.

For the person scenario, I'd suggest pessimist:

noun

1. a person who habitually sees or anticipates the worst or is disposed to be gloomy.

From Random House Dictionary

This person is known as a "naysayer"

"Naysayer" from M-W:

:a person who says something will not work or is not possible : a person who denies, refuses, or opposes something

Some great synonyms from Thesaurus.com include:

• wet blanket
• party pooper
• prophet of doom
• killjoy
• sourpuss