I've always thought the amount of time Darcy gives to addressing splitting up Bingley and Jane awful for Lizzy as it is a further slight to Lizzy.

But your comments have made me consider this a little further, and I think the letter addresses the slights to Darcy's honour as he perceives them.

Darcy has less argument regarding his behaviour towards Jane and Bingley as Darcy does not believe he behaved dishonourably. Darcy impartially considered the facts and acted on them. It's disappointing, however, that there was no attempt to ameliorate the situation eg an apology.

Lizzy's comments regarding Wickham and Darcy are entirely untrue, and Darcy provides detailed information regarding why this is the case. In the course of the letter, Darcy provides an insight into his personal standards and makes it clear that he shares Lizzy's negative view of the behaviour attributed to him. If Darcy is to be believed, the end of the letter shows Lizzy that the facts have been misrepresented to her.