I do trust the police.
But as said above, with any large-scale organisation, there will be less-than-trustworthy components. Temptation is there, whether it's backhanders, outright bribing, access to drugs, etc. There are bound to be some who go beyond the boundaries and break the rules (and law) to get ahead, or to get what they want.
And with anything, in general, power corrupts, so those at the top have more to gain (or maintain) by acting on the margins of what is acceptable, and indeed legal.