This is one of the reasons vigilantism works better in fiction than in real life. In cases where some vigilante left a beat up suspect and some sort of evidence, any competent defense attorney is going to move to have the evidence suppressed due to issues around chain of custody and possible tampering. They would likely also push the narrative that the vigilante is the real criminal and left the evidence to frame their client. Between possibly getting much of the evidence suppressed, and building doubt around anything remaining, a conviction could be really hard for the prosecutor.

This also ignores issues around vigilantes going after the wrong person for something (see: lynchings) and applying wildly disproportionate, extra-judicial punishments for crimes (see: lynchings, again). Crime and punishment really are hard problems which don’t lend themselves to easy answers. And there is a reason the Code of Hammurabi is seen as such a big deal in history. Rule of Law is an important concept which protects people.