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.)