A lot of you folks are missing the point the OP is trying to make...
If you are truly an Atheist, and thus believe a human is just another species on Earth, and ultimately like any other species, then nothing you do or don't do ultimately matters...
If you are truly an Atheist then morals and ethics, and being a "good" person shouldn't have any meaning to you...
Ultimately, it's survival of the fittest...You are "good" if it helps in one instance and "bad" if it helps you in another...You shouldn't care about being "good" all the time...For what...?
If you are an Atheist, then why the hell should you care about "legacy"...? Humans care about what we do, because we have a MEMORY...At the end of the day, we want to remember, and we get pleasure or pain from what we remember and how we remember it...
Just an example, some people want to have children because the thought of having a "legacy" is appealing...However, that thought is only appealing because you have a brain that is functioning, and has be socially programmed to accept that kind of thinking...
But ultimately, we all die, and thus, we wont have any thoughts, subsequently, we wont be able to experience pain or pleasure...
Therefore, at the end of the day it doesn't matter if you leave a legacy behind, because you will die, and your brain will not be able to think or reflect on anything you did in your life...
Basically, it doesn't matter if you were poor or a billionaire, because once you are dead ALL experience is GONE...
So, if you are truly an atheist then it shouldn't matter if you "commit that" today or live your life until you are 100 years old, because at the end of it all, you wont even know you were ever alive...
To conclude, let me make this analogy...
If you enrolled in a course in University/College and the Professor told you on the very first day, "it doesn't matter how much you struggle or enjoy the course, in the end of the semester, you are GUARANTEED that you are going to fail, and even worse, you wont even remember the experience of taking that course"
Why would anybody care about the course knowing and believing this...?