You could circumvent the problem by merely pluralizing the subject:

It is unbelievable how perpetrators will cast themselves in the role of victim.

This is what I would do in this case, as neither oneself, themselves, or themself are strictly correct here, and as there is no such thing in English as a singular gender neutral reflexive pronoun.

You could circumvent the problem and avoid overcomplexity by merely pluralizing the subject:

It is unbelievable how perpetrators will cast themselves in the role of victim.

This is what I would do in this case, as neither oneself, themselves, or themself are widely endorsed here, and as there is no such thing in English as a singular gender neutral reflexive pronoun.

You could circumvent the problem by merely pluralizing the subject:

It is unbelievable how perpetrators will cast themselves in the role of victim.

You could circumvent the problem by merely pluralizing the subject:

It is unbelievable how perpetrators will cast themselves in the role of victim.

This is what I would do in this case, as neither oneself, themselves, or themself are strictly correct here, and as there is no such thing in English as a singular gender neutral reflexive pronoun.