I think that there is always room for improvement in any screening process; they will never be perfect. It is the reason that there are spies from other countries who conduct espionage in the United States, the reason some employees are later on released by their employers, and so on. With that being said, while I said "agree" above, I think that, like with the U.S. intelligence community and with various employers in corporate America, the "bad eggs" are few and far between.
I believe that as members of the police force are given firearms, there should be a stricter screening process on potential members. Any individual who has a vendetta against a particular group of people would have the power to severely harm them. I believe if we are to be able to trust the police force, they must have good intentions and be just people.
Yes, we need stricter screening on police force prospects because even police officers are capable of committing crimes. In America today, there have been a lot of cases of police officers shooting unarmed individuals and abusing their power. More needs to be done to ensure that police officers have a clean past and are genuinely good and honest people.
Candidates for the police in the United States are subject to the same kind of stringent screening processes that we can also see in the rest of the industrialised world. Endemic problems with the police force are more to do with structural issues rather than specific 'rogue' individuals and so it is those systemic problems that should really be addressed.