The basis of existentialism, for me, is the realisation that all that we can know is our experience plus the realisation (I would say it should in fact be called a belief/assumption) that we have free will. It then takes this realisation out into the world and, well I don't know what it does then. I've always thought of ethics as a bit of a woolly appendage to philosophy.
For the most part, once I get to this stage, I don't look at the situation from that point of view at all. I find, very often, that an outside-in perspective that looks at biology, evolution and history can provide excellent insights into the question of how an individual should act within a society. Politically, individual freedom except where it impinges on the freedoms of others, in which case, a settlement – a social contract – will need to be negotiated. Collective provision where possible without impinging on freedom (and I do not mean the economic 'freedom' to take a larger share of the pie for yourself here – that's already disqualified as it impinges on the freedom of others to have their share), mutual aid as the very best way for us to get along.
I don't need the existentialist's point of view to reach these conclusions. There are other routes to them.
That was my point really, at the beginning, that existentialism was something to get out of your system. It is a dead end. It does not lead to useful answers to these questions. It leads to the Outsider. It leads to Raskolnikov. It leads to despair among some. But its philosophical underpinnings are (almost) sound, and it is a good way to discard the garbage of superstition and irrationality.