Because every single paedophile is different. Like murderers, there are different degrees.
Some paedophiles know that what they want is terribly wrong and make sure that they receive professional help.
They never allow themselves to be alone near children. They never look at child porn. And throughout their lives, they never, ever touch or harm a child in any way. Should they be murdered too?
Others actually commit serious offences against children, knowing full well that it's evil, but they don't care. Maybe they should, IF convicted in a court, be euthanised.
But no civilised country would allow any member(s) of the public to murder anyone at all. The law has to be followed, or we'd end up with innocent people being lynched or whatever all over the place. People can be very stupid, especially when in company with other stupid people.
I heard of one group which attacked a house, smashed a car, and tried to burn both because the owner's office was in that house and one of them saw his professional sign on the gate, which said "Paediatrician". They thought it meant the same as 'paedophile'. Yes - really.