This saying is not one that's very easy to fit into a framing sentence, but it's excellent to use as a stand alone proverb or expression. It works as a warning or an admonition to people who are considering doing the wrong things for the wrong reasons, but it works even better when those people are considering doing the wrong things for the right reasons.
1. The righteous never stray from the path
2. The ends don't justify the means
The kanji read as follows: bad tree steal spring (spring as in water, not the season) and when I read it I thought maybe it meant that "The bad tree steals the water from the spring," implying some kind of inverse lesson about how we should strive to not be like the "bad tree," but as I've learned the hard way, these kinds of speculative inferences about why something means what it does in Japanese are RARELY accurate. A little bit of web-research yielded the true origins.
This site, despite being one big visual headache, actual contains quite a bit of useful information about Japanese expressions. In fact, it might contain all of it. It says, if you can track down the section on 悪木盗泉, that it's a warning. No matter how hot it is, one should never sit in the shade of the bad trees; No matter how thirsty you are, there is no cause to drink from the stolen spring. It seems more poetic that way, doesn't it?
There are even some other similar expressions that make use of the bad tree imagery: 「熱しても悪木の陰に休まず」 or just ｢悪僕之陰」will do the trick.
I guess, conceptually, we have some similar sayings in English. "The road to hell is paved with good intentions," springs to mind. And with the Japanese cultural emphasis on ideas of karma (因果応報), you can imagine how an evil deed performed by a good man might negate his goodness, or how a noble deed accomplished by immoral means can't really be counted as a noble deed.
I've found some other examples of uses of 悪木盗泉 that prescribe a がんばって、ガマンして code of living, like on this site, SomeGirls_blog, run by a guy who seems to be very 男ポイ. He seems to be lamenting a female friends impending divorce (or horrible marriage), but here's what he says the 悪木盗泉の心構え male is responsible for:
男なら、どんなにプレッシャーが掛かっても前に進まないとね。Sounds a bit like a martyr-complex to me. I wonder what a woman's work consists of?
A man must press on, no matter how much pressure he is subjected to.
A man must work to make the people in his life happy, no matter what difficulties this may entail.
A man must be strict and relentless in his own discipline so that he never makes any missteps, never strays from the path.
A man is responsible for himself, and even if he is all alone, he must make sure that all of his actions are just.
The company president is up to some shady dealings, and even if it means my neck, I'm gonna keep on the righteous path and go public with it.