There is nothing to indicate he was attempting to or planning to take the surviving twin other than Monica saying he came back for Carter and then got “spooked” and left. I’ve watched the video way too many times, he left without any attempt at taking Carter and clearly wasn’t spooked. JMO IMO
He had a short bladed object - get real, a kife - that he was flipping around in his right hand. I have little doubt he was considering killing the other child. Whether he was interrupted or spooked or the voices in his head had other plans is not really possible to say, but he did leave abruptly.