I am sorry that you feel so discouraged at this early stage. Marek's is pretty devastating when it first happens and a lot of what you read is pretty scary (in my experience not nearly as bad in reality), but it is manageable. Sadly illness and death is an inescapable part of the chicken keeping process as they are prone to so many ailments and diseases as well as predation, but there is also much joy and the "miracle" of eggs of course from keeping them, to offset the heartache! The bad times sometimes make it hard to remember the pleasure that chickens bring and unfortunately you have hit a bad time early on in the experience but there will be more good times to come.
I cannot tell you what the right thing to do is as regards this chick or future chicks as only you can make that decision based on research and gut instinct and we all have different approaches, but I do sympathise and wish you better luck in whatever you decide.