>Even ignoring the many physical dangers of such an act, if the person “arrested” doesn’t get convicted (also pretty sure it has to be a felony conviction, at least here in MA, though IANAL), it opens you up to a false arrest, or possibly a false imprisonment, charge.

In England and Wales you merely need a reasonable belief, the person not being convicted does not automatically open you up to trouble any more than it would open up a police officer to trouble.

https://www.legislation.gov.uk/ukpga/1984/60/section/24A