Evidence is irrelevant to Dershowitz once he agrees to take the case. His job is to get him acquitted. If you have a conscience you shouldn’t be a defense lawyer.
I used to think the same way as you, but a defense lawyer could someday save you from being mistakenly thrown in jail or even sent to the electric chair. A defense lawyer's job is to ensure a prosecutor proves his/her case beyond a shadow of a doubt regardless if the lawyer thinks you are guilty or innocent. Without defense lawyers, there would be many more innocent people in jail or worst, unjustly executed.