Someone once asked...
Someone answered with kindness...
because it's not fair. they might not know that awful person well enough. or they don't want to ruin their legacy. or they never thought of such a "revenge". or they might not have the opportunity to do so.
Another soul answered...
This would be rationally aligned with their set of beliefs in the moment - they do not do this because they are very unwell. They are generally not soldiers of a cause capable of reasoning, however lucid they seem - they need medical help.