Honestly, vigilante justice is something that can rarely be justified.
What's even less justifiable is such a long sentence when no one was killed, he clearly feels remorse for what he did, and this is possibly the most obvious case of a mentally ill person who's actions can be directly traced to his abuse as a child.
While it's true that rehabilitative justice is something unheard of in most of the world, and it doesn't work for everyone, there are better ways than a 28 year life-ending sentence, because let's not fool ourselves, prisons are a punishment, not a path to rehabilitation.
But I'm not even going to pretend that I feel bad for pedophiles that end up beaten, killed in prison, or murdered by the parents of the victim. Which happens quite regularly since most people understand that this is one of the most heinous crimes that can be committed against an innocent person.
Rape and continued abuse, specially as a child is something that will be a permanent wound in the mind of most people who suffered from it, many end up living a broken life, some will ruin the lives of others since this is a cycle that often turns victims into abusers, and some of them will end up taking their lives because of it.
I should know, I was a victim of something like this and have talked to many people who shared similar tragedies. Even though this is not even close to being my main reason, and I've moved on, I still believe that punishments for rapists should be harsher, vigilantes shouldn't feel like doing what the police and court system refuse to do.