His argument stripped down is:

(1) All cops agree to enforce all laws.

(2) Many of those laws are bad.

Therefore,

(3) Every cop enforces bad laws.

Therefore,

(4) Every cop is not good.

It's not a good argument. (1) seems plausible as being true at first, but then you realize that there exist cops that do not enforce some laws, e.g. crooked cops that might look the way other because they are on the take. People will say this supports Higgs' conclusion in (4), but it doesn't. Instead, it undermines his inference from (3) to (4). He wants to generate "badness" from support from bad laws; not from the existence of non-good cops (otherwise he'd be making a vacuous argument that non-good cops implies non-good cops). The falsity of (1) undermines the way he generates the existence of "no good cops". That's the first problem.

There are also a number of problems with (2). Firstly, it is unsupported. Which laws? Where are the supporting arguments that "many" of them are "unjust" and "wicked"? Secondly, he implicitly rules out non-bad laws. This could run the spectrum from good laws (not raping and torturing Dr. Higgs' family members) to neutral ones. Clearly these laws exist, since he says "many" laws and doesn't quantify over all of them. Given the laws are doing the generation of good and bad in (4), then this would imply the existence of neutral or good cops. In fact, this is a counterexample to the validity of his overall argument: the existence of non-bad cops.

There's also an un-argued part of his reasoning from (3) to (4). He shifts from attributing an evaluative property of laws to an evaluative property to cops. For this inference to work there must be some sort of suppressed moral principle between (3) and (4) such as "if someone enforces something bad, then that person is bad." It's pretty easy to generate a counter-example to this principle: someone enforcing something bad that happens to not be a bad person. History is filled with people that thought that they were good or even neutral (both individually, within their wider social circle/norms, and having what they thought to be theoretical justifications), and enforced extremely bad things, e.g. religious and political fanatics. As said above, one might spin this to support his argument; but it doesn't, since he's arguing about a structural mechanism that attributes badness of a group from badness of law.