Ian Knight's assessment in Zulu Rising repays some study. The following is my understanding of what IK is saying (so any misrepresentations are mine and not IK's).
We know that Chelmsford's plan was to engage the Matshanas before moving on to the main impi (that is what he describes in his orders to Durnford). It is a further stage in his mopping up of the local chiefs, as he had done with Sihayo a few days before. He even has some thoughts that Matshana may surrender rather than fight. He sends two of his own ADCs (Gossett and Buller) out with Dartnell on his reconnaissance and they report back to Chelmsford after Dartnell encounters the Zulus. There was nothing in their reports to alarm Chelmsford and he sends out some further mounted infantry and pack horses with extra provisions. He gives an order that Dartnell is to attack the Zulus when he thinks fit.
The second message arrives from Dartnell during the night reporting increased numbers of Zulus and saying it would not be prudent to attack them without some white troops. Dartnell's substantial NNC contingent had been spooked during the night and, understandably, he could not rely on them with just his 70 or so Natal Mounted Police and Volunteers.
So why did Chelmsford take such a large proportion of his force out with him to join with Dartnell?
I like IK's reasoning a little better than the labels. It had not been unusual for Chelmsford to take substantial numbers of troops out on sweeps of the bush on the Eastern Cape Frontier to flush out the enemy and bring them to battle. Wood was, at that very time, doing something very similar 50kms to the north. And Chelmsford now has an inkling that a much larger Zulu force is in the vicinity and he fears that if he hunkers down his main force at Isandhlwana the main Impi will by-pass him without coming to battle and head direct for Natal. Each of those sets of reasoning are plausible and have some merit. To my mind they have nothing to do with arrogance or complacency. He was of course proved wrong for reasons we have already alluded to - but he was not the incompetent that we keep labelling him as.