I get what you're saying, but if you're looking at this from such a broad, interlocking perspective, wherein every act of good inevitably brings about bad consequences, wouldn't it be also true that for every bad there is also an inadvertent good that comes about too?
In the end, life--and indeed nature-- is just the check and balance of various consequences. We assign certain values to consequences and develop creeds which dictate our actions; we may resolve to do more to bring about x and to counterpoint y, but whether the intended or inadvertent outcome is positive or negative largely depends on the perspective you're viewing it from. There's no true inherent value in anything that we do. What is good and what is bad is entirely limited to your perspective... which by the way, happens to evolve from and build upon the perspectives of others.
If we accept that good and bad are just values, and we accept that there is a ripple effect of our actions that result in numerous outcomes that can be labelled "good" and "bad" depending on perspective, then there is no absolute good and there is no absolute bad. Therefore, depending how you want to look at it, yes, it's impossible to do all good... but it's just as impossible to do all bad.
Free from the burden of that absolutism, we have a choice. Do we sit around and do nothing because no matter what we do we're going to fuck up somehow, or do we take deliberate action that we know and believe will directly benefit someone and trust that, even if there are inadvertent bad outcomes, there will also be inadvertent good outcomes in addition to the intentional good that we set out to do.