A lowlife is a no-good bum or a criminal, someone who deserves contempt. A car thief and a cruel bully could both be described as lowlifes.
Calling someone a lowlife is an insult — a lowlife has a terrible, mean or criminal reputation. You should probably save this word for the worst of the worst, like animal abusers and scam artists who steal money from elderly people. Lowlife started out as an adjective meaning "disreputable," and began to also mean "no-good person" in the early part of the 20th century.