First, I think it is totally cool to want to go to law school simply for the money. Also, as someone else pointed out, there are very few jobs in public interest, so most of us will not be working to "change the world" after we graduate.
I'd like to add something to the "mercenary" idea.
I was speaking with a friend - he's an attorney. We were discussing this topic and he brought something to my attention. He said after the time he spent in law school and his experience as an associate, you really start to think differently than you ever had in your life - and you analyze every argument in a "lawyerlike" manner. Thus, you begin to see the pros and cons of both sides of the argument, and it becomes less a moral issue and more of a legal argument battle of wits. So, it's not as if you sold your soul - unless you're defending genocide. There are always two (or more) sides to an issue. We just fail to comprehend the other side(s) in our everyday lives.
I believe that this process does happen. However, this is not necessarily the final stage.
Normal humans go through stages of moral development. I personally find Kohlberg's stages compelling. http://www.nd.edu/~rbarger/kohlberg.html
There is actually a stage called “Law and Order” that he believes most adults stay in. This stage is characterized by an orientation towards duty or obligation.
If you take an adult that is in this stage and teach him law, then you did not evolve or devolve him in terms of moral behavior. Because the same group of laws can often be applied to come to mutually exclusive positions, the lawyer can use his new skills to destroy the framework of the “Law and Order” moral stage.
Let me pause for a moment and make some points. I do not personally believe that someone in stage 5 or 6 is somehow superior to someone in the “Law and Order” stage. So, please do not attack that. Also, I do not think that my above hypothetical lawyer is inferior to any other person. I merely think that this model explains the attitude described by Bahamut.
We tend to move to the next moral stage by learning from others. Since there are more people that do not think like lawyers, I think non-lawyers have more opportunities to advance to the stage beyond the “Law and Order” stage. On the other hand, I think the analytical and critical thinking skills that successful lawyers have would make it easier to process the information necessary to move to the next level.
Overall, I think law school has little impact on whether someone progress past the typical views of morality. Although, I do agree that most lawyers exist in a stage parallel to the typical “Law and Order” adults.