The word to describe such a loser who ruins everything

I'm sure you're not looking for party pooper, how about dramaqueen?

Malcontent may work in the context provided. Killjoy is another possibility.

Is the person ruining things on purpose, or is it simply his presence that spoils the event?

Buzzkill...

How about "Jerk"?

I agree with ehBeth's question, re the subtlety..

in the meantime, I'll add spoiler.
Oh, that was before I saw her mention the word, but it doesn't matter, same thinking.

wet blanket

person from Porlock

that's good, I agree with that.

Spoiler is a colder and meaner word to me.

I suppose I should mention the wordage, **** up - but that is finally not very interesting, nor helpfully descriptive.

In my day we called people like that a "Jonah". I believe there is a Yiddish word also. Most cultures have the concept.

Don't know Yiddish spelling. phonetically something like malkmudis
Angel of Death

A boor.

A stick in the mud or a spoilsport.

The latter.

Excellet guys!
Thank you all.

I believe the term "a Jonah" "(note the capital J) derives from the person of that name in a story in the Christian Bible. A long-established expression among sailors uses the term to mean a person (either a sailor or a passenger) whose presence on board brings bad luck and endangers the ship.

Later on, this meaning was extended to "a person who carries a jinx, one who will bring bad luck to any enterprise." An example would be Samuel Taylor Coleridge's Ancient Mariner, who was cursed after he killed an albatross, an act once held by superstitious sailors to bring bad luck. An albatross following a ship was held to bring good luck on her and those who sailed in her, and killing the bird not only cancelled the good luck, it actually brought bad luck.

Such a person, therefore, is as much the victim of a curse as the people who come into contact with him and suffer as a result.

Jinx! I should have thought of that.

If that is common usage, I don't know about it.

I'm sure there are pretty good phrases for this, some maybe from good novels.

Well, it has been pretty common usage in Britain all my life, and I am no spring chicken.