A "harasser" is someone "who makes others feel uncomfortable [by] using words", gestures, and touch.
A "stalker" is someone "who makes others feel uncomfortable" by persistently trying to interact with them, either by using information they thought was private, and/or after they had told the stalker to have no further contact with them.
As Arrowfar suggested, "lowlife" is an offensive term for a poor person who might have criminal intentions.
"Crook" is a mildly offensive term for a criminal. For example, when Richard Nixon said, "I am not a crook", it immediately made many people think he was a "lowlife" and unworthy of being President.
"Mugger" is a factual (but still insulting) term for a criminal who robs people on the street or in alleys by force (or threat of force).
"Thug" is an offensive term for an intimidating, possibly criminal, person. Many bodyguards and muggers are called thugs.
"Rapist" and "child molester" are factual terms for people who commit particular crimes. Because these crimes are so abhorrent (especially to many imprisoned criminals), they are very insulting terms. (Many criminals consider it honorable to harm or kill rapists and child molesters.)
In this context, I think two words would work.
Instigator - a person who causes something to happen, especially something bad
or
Inflammatory person
As we use 'inflammatory remarks' to denote something that causes something bad, inflammatory person is the one who causes dispute by adding fuel to the fire
Best Answer
You would use "master of ceremonies" or "emcee." The word "emcee" comes from the pronunciation of the abbreviation of master of ceremonies, "MC." Another less specific and casual option is "host."