What puts it in that category while other flaws or actions can be forgiven?
The only unforgivable sin is the sin against the Holy Spirit.
And the sin against the Holy Spirit is despair.
However the sin of accidie is akin to despair.
And unfortunately the sin of accidie is expressed today in depression.
And depression is one of the most common sins of today.
But the greatest trick pulled by Satan was to persuade us that depression is not a sin.
And so it has become an epidemic.
Even the DSM-IV says that Depression is accompanied by delusions. But Depression is accompanied by the most subtle of delusions, and they are emotional delusions.
And emotional delusiions are so subtle they are easy to deny. And how natural it is for a depressed person to react violently to be told they are suffering from a delusion. No matter that delusions are real - they are real delusions.
And of course the essence of sin is deception and denial.
Sin is so ugly it is necessary to deny its very existence.
But all sins can be forgiven except despair because the despairing refuse to ask for forgiveness.
It would depend on the reasons. Would I murder a 6 year old boy cause he robbed my store of roughly 5 dollars of chocolate, no. Would I murder (and quite slowly) the man who rapes said boy, every day of the week.