It is possible to be of the view that the death of a pedophile is not a great loss to society, but at the same time thinking that the person who killed him should be held responsible for their actions and not upheld as a hero.
It is also possible to not judge someone for killing someone based on their reasons while at the same time not congratulating them or feeling that what they did was a good thing.
Murder is wrong. It doesn't matter why you kill someone, if you intentionally hurt someone and they end up dead, then you were in the wrong for doing so. And this woman went to the house of this man armed with a knife - the fact that the jury seemingly believe she didn't mean to cause him harm is pure luck on her part. Personally I don't believe she didn't intend to cause him harm, even if she didn't intend to kill him. But two wrongs don't make a right, and regardless of what this man did, she was the one who held the knife and then stabbed him, not once, but eight times. Of course she has to be held accountable for that.
We don't have the death penalty in this country, and as such we don't permit the public to kill people whose sentences they don't agree with. trial is by judge and jury, and legal process. Convictions are based on evidence not emotion or supposition. To allow otherwise is the slippery slope.
Whether this man's previous sentences were adequate is a whole different debate. but if people feel strongly about the lengths of sentencing for child abusers/murderers/perpitrators of other crimes then there are avenues they can go down to hope to effect change. Campaigning, petitioning, speaking to MP's. Not going to the house of someone whose sentence you feel was wrong and stabbing them to death. Where does that end? If we ended up with longer sentences for pedophiles what happens then when someone decides that custodial isn't long enough and that actually we should re-introduce capital punishment? Would it be ok for someone to go out and kill someone on the basis they will only get life and they feel that's not enough?
There will always be the cases where there is some mitigation, and in this instance we don't know what that mitigation might be. We can assume but we don't know. But on the whole if people start turning pedo killers into heroes then we do open up permission for vijilanti justice if no action is taken.
And while this woman might have been compelled to kill this man, the fact is that she has contributed to the suffering of the children she sought to protect by doing so. Yes, the urge might be there to kill the bastard, but now he will A, never get a trial so the children he abused will never see justice, and B, if they were her children, they will not only have to come to terms with what happened to them, but the one person they should have been able to trust to be there for them won't be, because she didn't have any self control.