There are plenty of non-profits that team with law enforcement for this cause. They should join one of those.
Vigilante justice isn’t something advance societies should advocate and regardless of the crime, due process and assumption of innocence still mean something.
Would evidence not acquired by law enforcement hold up in court? Most likely not. So this freak would get let out based on a technicality, able to threaten more children.
Vigilante justice marks a point the state has failed at......often numerous times