In Marxist-Leninist theory, the lumpenproletariat is a non-refolitionary and often counter-revolutionary class which is comprised of criminals, thieves, drug addicts, prostitutes, and homeless people. This class is the most ripe for opportunism since they have little left to lose and everything to gain– but in the face of this simple fact, they are more willing to serve the bourgeoisie against the proletariat because they'll get paid.
In early capitalism, according to Friedrich Engels, the lumpenproletariat were far more likely to join the proletarian movement behind the proletariat, but as capitalism became more ruthless and monopolized they sunk deeper into violent criminality, addiction, abuse, and other degenerative and destructive behaviors.
For anarchists the lumpenproletariat is the revolutionary subject rather than the proletariat, which leads anarchists and their allies to serve the lumpen almost exclusively, and make them the targets of their propaganda campaigns. This treatment of the lumpenproletariat by the anarchists leads to cliques of decadent, counter-revolutionary, violent, criminals, who are so thoroughly opportunistic that the bourgeoisie easily manipulates them through promise of money or power.
When the lumpenproletariat thus becomes "political" and "revolutionary" on behalf of capital, the result is fascism, "the open terrorist dictatorship of capital", as Georgi Dimitrov defined it. One example of such historical development was the Iron Fist organization in the Weimar Republic, which was made up of violent criminals, pedophiles, drug addicts, and street brawlers and later became the Nazi Party. Another such example was Mussolini, who was inspired by his father to become an anarchist, and soon thereafter became an agent of MI6, a street thug and violent criminal, a columnist in support of anarchism and terrorism, and leader and founder of an armed anarchist group which grew into the National Fascist Party. Other such examples can be found on the anarchism page.