I remember my mother saying, decades ago, that if someone killed one of her children (I guess this was in response to something in the news then about a child having been killed) that she would want to find the person responsible and kill him/her herself. But that she would also 'expect' the state to stop her from doing that.
I remember that very clearly; don't know why, as my mother and I generally had diametrically opposed views on all sorts of things.
So I'm with Georgimama, WinkyWinkola, etc. here. Yes, of course if someone threw acid in my eyes I would want to do the same to him.
But my wish to be vengeful, however understandable, wouldn't necessarily be right in terms of societal justice. (And I know that this is easy to say, since it hasn't actually happened to me or to someone that I love.)
As to whether or not violent criminals receive what many of us might consider 'adequate' sentences - well, that's a different argument, isn't it?