I assume you are speaking of 18 USC 242:
http://www.law.cornell.edu/uscode/se...2----000-.html
The key element being:
Whoever, under color of any law, statute, ordinance, regulation, or custom, willfully subjects...
Police enjoy qualified immunity in civil suits, a heavy burden to overcome.
If such criminal charge were brought, then "beyond a reasonable doubt" would be the burden of proof.

