I don't see a motivation that the Kerchers would be motivated to harm Amanda Knox other than their belief that she killed their daughter. In this context, what do you think is the underlying motive that they are engaging in self-deception to accomplish?

They -- and their lawyer -- have a pecuniary incentive to seek a verdict against Knox and Sollecito, since it would entail the imposition of monetary damages in the millions of euros. Needless to say, Rudy Guédé's financial rescources are almost certainly not comparable to those of Knox and Sollecito (even though the latter two aren't themselves extraordinarily wealthy).

Of course, this probably doesn't directly pass into their conscious motivation; but it still likely affects their judgement.