The next paragraph is

After a few minutes reflection, however, she continued -- "I do remember his boasting one day, at Netherfield, of the implacability of his resentments, of his having an unforgiving temper. His disposition must be dreadful." (my emphasis - G)

I don't think that Darcy boasted about this. This is what he said:

"No," said Darcy, "I have made no such pretension. I have faults enough, but they are not, I hope, of understanding. My temper I dare not vouch for. It is, I believe, too little yielding -- certainly too little for the convenience of the world. I cannot forget the follies and vices of others so soon as I ought, nor their offences against myself. My feelings are not puffed about with every attempt to move them. My temper would perhaps be called resentful. My good opinion once lost is lost for ever." (Ch. 11)