Like if you know some cop was just being a douche bag do you still have to try the case as an associate DA (assistant state attorney in FL)? Do you as an assistant/associate prosecutor have the power to decide what cases you try?

Plea bargains are often utilized. Discretion probably varies by years of experience & latitude offered by one's supervising attorney.

P.S. Were the officer's actions illegal ? Thereby excluding crucial evidence ?

Most DA's offices will have an intake department, where they decide whether or not to take a case. As an ADA, it's then up to you to achieve the outcome that will best pad the stats of the DA's office.

ADA's usually have an oath to serve justice. In theory, they are supposed to adhere to that over winning at all costs. If you know there is exculpatory evidence, you talk to your supervisor and the defense attorney and work to get the charges dismissed. It is highly unethical to do otherwise.

I was in the Criminal Defense Clinic in law school. And I have a couple of friends who were/are prosecutors.