They falsely claimed he'd made an actionable threat. We can't remove their power to request warrants and arrest people for legitimately threatening others, right?
The magistrate judge is supposed to be a check on that power. Unfortunately, they've become rubber stamps for the most part. In a functioning system, the judge would block the warrant and arrest.
I think there are ways to have a system where judges do that, without having to criminally prosecute either cops or judges.
OP says that they left out information (which cops do all the time) but that the warrant shouldn't have been granted either way because of SCOTUS precedent.
Would welcome reform that makes it harder to lie on warrant affidavits, although again, that should be civil in nature.