This stuff comes out frequently in conversations about abuses of police power. The intent of it seems to be to deny that there is any systemic issue to address, only a few bad cops.
Let me try to steelman this:
"Inevitably in any organization as large as the police are nationwide, there will be individuals who misbehave. Their bad behavior might tarnish the reputation of police generally, but this is not fair because it is not an accurate reflection of the other cops. Other officers who are doing their job correctly should not be blamed for the ones who are doing theirs poorly, in the same sense that we don't blame all Christians for the small minority who are Christian terrorists."
This "a few bad apples" argument totally falls apart because police forces are hierarchical organizations. There is a chain of command. Regulations. Extensive and ongoing training. Review. Orders. Punitive actions. Other groups, like "Christians", do not contain this organizing structure, so there can be no accountability to the whole for the actions of a part. The group "Christians" do not have any real power over the group "Christian terrorists".
Police organizations do have real, extensive, and direct power over the police who work for them. An agency is responsible for how they respond to these incidents, and what we frequently see is agencies closing ranks around their officers to defend them from criticism of wrongdoing. So that, by itself, already makes this a systemic problem, and a serious one. In the "few bad apples" hypothesis, the police chief and all of that officer's peers are horrified by the wrongdoing, and massive social and institutional pressure is brought down from within to correct the issue by shaming, denouncing, remediating, firing, and/or arresting the offending officer. Is that what we observe?
Agencies train and indoctrinate their officers into an organizational culture; they set the standard for what is or is not acceptable. Because of that, there is a sense in which the chain of command is responsible the very moment one of their officers does something wrong. Obviously someone somewhere will still make a mistake, but in an organization where the standards are correctly communicated and upheld, the group will police itself at a peer level when possible and at a supervisory level when that fails. In an incident with 5 cops on scene, when one got unnecessarily violent, the others would recoil in horror and tell them to back off or tone it down. "You're out of line Officer Johnson, step back and we'll handle this. We can discuss it back at the precinct." Or similar. Is this what we observe?
In the best organizations a commander will take real responsibility for the failure, take real corrective action, and vow that the organization can and will do better in the future. Is this what we observe?
A better analogy than the Christian terrorists would be Catholic priests and cardinals. Is it appropriate to criticize the Church for the child rapes conducted by priests? You're damn right it is. In addition to having a responsibility to preventatively protect the children in their care, they have a responsibility to respond to incidents in a way that protects victims and others who might be vulnerable to future victimization. They have a responsibility to use their institutional power over the priests to hold them accountable for their actions, at minimum. The cardinals who knew about this and did nothing or protected the priests have no defense. The organization as a whole has no defense. The most I can say for the clergy in general is that it is plausible that many of them did not know this was going on. A priest cannot be responsible for his peers raping if he does not know it is occurring.
The same cannot be said for the police who were standing there, watching abusive violence from other cops. The same cannot be said for the police who were helping. The same cannot be said for the police who express public solidarity with these abusive officers. I actually think law enforcement, as a whole, is more culpable for their wrongs than the Catholic Church, because the wrongdoing of their peers is not secret. We are all aware, and still they close ranks.
I think part of the issue is that they (and we) have decided that a particular group, "criminals", deserves whatever they get in their contact with law enforcement, so once they have mentally designated someone as a trouble-maker the ethical concerns evaporate. This is why there is a rush to demonize the victims of police violence. We have accepted the premise that it is possible to deserve police abuse, that it is justifiable if we disapprove of the victim sufficiently. I won't go into it now. Suffice to say democracy cannot thrive when state agents decide who does or does not have rights, which is why the ACLU defends terrible people against infringements and why it must be that way. Anything else makes your rights up to someone else's discretion.
This does not mean that all cops are "bad". It does mean there is a systemic issue with abuse of power in American law enforcement. It does mean that there are law-abiding citizens with good cause to be afraid of the police. In my view, because of the social dynamics that arise in groups who rely on each other in life-threatening situations, this problem can only be addressed from the top down. Institutional reform with teeth is required. Officers must be held accountable for what they do, and what they accept from other officers. Whistleblowing must be a thing that is actually safe to do. We're a long ways from there.
They will not give up their power and privilege easily, will not break ranks quickly, will not accept real accountability if it can in any way be avoided. The powerful and exempt never do. Please don't help them avoid accountability by dismissing the problem as isolated to a few bad cops, because it isn't. That is not what we observe. And this is not what we deserve.