Why would I defend my client if I think he is guilty?
Because it's my job. My boss will kill me if I don't.
I'll get paid anyways. If we win the other part will pay my bill, and we lose my client would pay for it. There are no economic downsides to it.
I'm a lawyer, it's my proffesion -and vocation-. Does a surgeon leave someone die on their operation table just because he doesn't like him or her?
Laws specifically refer to the right of every individual to have a technical defense -a lawyer- on a trial or procedure. Moreover, the code of ethics settles the responsability of the lawyer to work for the best of its client. It is within our justice system and values as a society. Everyone has the right to be defended by a lawyer of profession.
Prejudice is a bad counsellor. My client might be as guilty as the other counterpart. In dubio pro reo, in dubio pro cliente.