Of course it's not "prettied-up social Darwinism", because libertarianism is fundamentally at its core merely "self ownership".  Self ownership is quite natural and good - especially among social creatures of all shapes and sizes that make subtle economic decisions to behave "selflessly" in return for community bonds that result in the security needed for self preservation.

A person who owns himself must agree that everyone else owns himself.  Those let down by society will seek to preserve themselves and their families in any way necessary.  To avoid such backlash, the only recourse is to ensure that everyone - no matter how helpless or anti-social – has recourse for survival.  Luckily we don’t have to devote a lot of logical thought to this – non sociopaths feel pity for the hopeless and try to help them in a variety of ways and to varying degrees based on an assessment of comparative wellbeing.

So no part of “self ownership” suggests that a person should let a poor or helpless person die in a gutter, yet it does raise the question of whether or not people are better off being cared for by their government or by charitable people in their community.  Charity is “the voluntary provision of money, materials, or help to people in need”.  People who own themselves should object to involuntary taking (ie, theft) of their money for any purpose.

So ultimately arguments between socialists and libertarians boil down to weather or not it’s ok to steal from people.