How do we ensure that when every time a situation like this comes up

This isn't a reasonable argument. Tigers, lions and bears eat babies. How do we ensure they don't?

How do we ensure your doctor doesn't rape and kill you?

How do we ensure that the local librarian won't beat your skull in with a book?

We can't always ensure things. But we can take precautions.

when they act with impunity what do you expect people to do

Just like in this video, people record. People witness and testify. Even body cameras now.

There are safeguards in place.

No system will ever be perfect, and there will always be fucked up people.

Arguing and fighting with a cop is fucking stupid.