In 19:14 it says "Do not place a stumbling block before a blind man... and fear God..". Rashi explains that the reason it says here "Fear God" is because people cannot really know what your real intentions are - whether to help him or to help yourself. Only if you "fear God" will you definitely be honest and give this person the appropriate advice for him, as Hashem knows what your true intentions are.
The Torah recognizes that people have personal interests and they sway a persons actions. People are not perfect. People will falter in their decision making, because they will often put their own interests first.
A person has to be ehrlich. When the situation arises in which a person has this conflict where he can manipulate it to attain his own goals, he has to be extra careful and ensure he made hsi decision for the right reasons and with the right motivations.