"We are not going to build a mass movement by telling people “leftists are opposed to things that help you right now.”

This is the crux of the issue. The people in the United States who belong to a class that the police exist to help will always be counterrevolutionary, because the police do not exist to prevent violence. The police exist to perform violence to one class on behalf of another. This is the primary purpose of a state. A mass movement of propertied settlers whose interests the police represent is an obstacle, not a prerequisite, to proletarian revolution. Saying that we should empower US cops because some individual action might incidentally "stop a bad guy" is akin to saying that we should empower the US military because they built schools in Iraq, or a "floating aid platform" in Gaza, or "stop terrorism". It's not what they're for. It's true that many propertied Americans believe the police and military are there to help them, because the US is the imperial core and its citizens largely labor aristocrats. Socialism requires defeating those people, not persuading them. It requires organizing the subjects of imperial violence, not finding common cause with the beneficiaries of it.

The people in the US that require organization already understand on a deeply personal level that the police do not exist to protect them. They don't believe this because someone convinced them of it, they understand this from experience because the US has never afforded them the luxury of misunderstanding how quickly and easily cops are will kill them.