Life is impossible to get right. We are weak, we are wicked, we err.
But, fortunately, life is not brittle - it is not all or nothing, it is not a matter of either perfection or damnation...
because of repentance.
To repent in the name of Christ (because unless repentance is by Christ then it is just psychology) is to transform the basis of life.
Therefore our own sins are not fatal in an ultimate sense - so long as they are repented.
And therefore the worst of sins is to encourage sin in others - because while we are responsible for our own repentance or lack of it - others may not repent the sins we have encouraged in them.
More exactly, the worst sin is to propagate a 'world view' which denies the necessity of repentance for what really are sins.
This worst sin has various forms: advocating sin - depicting sin as fun or cool; advocating inversion of the Good - depicting Good as evil, and evil as the real Good; denying the reality of the distinction between Good and evil - between creation and destruction; between Love and hate;
...and inducing despair: to induce despair in another person is indeed a terrible sin - because despair is loss of hope, and loss of hope prevents repentance (since despair tells us there is 'no point' in repenting).
Thus to advocate nihilism, the deny the reality of the real, to argue that life is nothing-but, that the bottom-line is meaningless, that purpose is illusory, that we are on-our-own... these are among the worst of sins.
To feel this way oneself is NOT such a bad thing, normal 'everyday sin'. To feel nihilism is just part of the human condition - something we may 'sincerely' feel, but which we can choose to repent in a trice.
But to advocate nihilism, deliberately to induce despair, to persuade another person to become a nihilist... well, that is a terrible sin, a sin of the utmost gravity.
Understanding that the worst sin is to propagate sin is to understand that the modern world is the worst society ever in the history of the world; and that the focus of evil activity is the Mass Media, and those who operate within it to propagate sin in all the ways previously described - and in particular, by direct and indirect means, tactically and strategically, carelessly and deliberately: to induce despair.