An entry on the Cambridge Dictionary reads:
lowlifes a person who exists by criminal activities or has a way of life that most people disapprove of.
So, hustlers or drug addicts could be referred as lowlifes (note - lowlifes, not lowlives) in disapproving comments, for instance.
If only disapproving comments it would mean e.g. Degenerado, marginal (aqui à margem da sociedade), but if there´s the criminal component, we could translate as "crimininal" (criminoso) thug (bandido/criminoso/marginal [no sentido de bandido mesmo, alguém envolvido com crimes ou contravenções).
So, low-life can have a "more strong" or "softer" meaning, depending on what the writer/speaker mean, or to whom he refers to.