Karl Marx had a word for the “underclass” as you describe them. That is the “lumen-proletariat” (i.e. under the proletariat). What he advocated for them was as bad (if not worse) as his policies against the bourgeois.
No. OK this is clearly an argument from ignorance. First, please don't give German lessons if you don't speak German. The word is "lumpenproletariat" and it doesn't refer to anything like the "underclass" of which I was speaking. Lumpen doesn't translate to "under the proletariat" but to something like "rags of the proletariat." Marx was describing the people who he saw as being the refuse of the society: thieves, con-men, pimps (brothel owners).
From Marx himself:
Das Lumpenproletariat, diese passive Verfaulung der untersten Schichten der alten Gesellschaft, wird durch eine proletarische Revolution stellenweise in die Bewegung hineingeschleudert, seiner ganzen Lebenslage nach wird es bereitwilliger sein, sich zu reaktionären Umtrieben erkaufen zu lassen.
Roughly translated to[1]:
The “dangerous class”, the social scum, that passively rotting mass thrown off by the lowest layers of the old society, may, here and there, be swept into the movement by a proletarian revolution; its conditions of life, however, prepare it far more for the part of a bribed tool of reactionary intrigue.
You'll notice that he basically said they were tools (ostensibly independent) who were likely to defend the status quo and would have to be defeated when the time came. I'd like to see where he advocated their slaughter.
[1] I'm lazy: http://www.marxists.org/archive/marx/works/1848/communist-ma...