I also think he wasn't thinking. He's a man of action anyway not a deep thinker and planner, is my opinion.
I also don't think he intended to kill her but acted out some anger he had - for whatever reason - and wanted to scare the parents, or people on the estate in general? It could have been random, maybe he knew that April would be the most willing to get into the car, maybe she had expressed enthusiasm about riding in it before. Maybe he just waved to the two girls and April was the only one to come over.
He could have had some "I'll show them" moment. "I take April and hide her and make them all scared and in the morning I bring her back. They won't/can't do anything, I'm family after all, I don't do anything wrong."
And then something went wrong and far beyond what he had thought he'd do.
I think he brought her to his home initally and it was there that something happened. Maybe April cried and he got inpatient with her, lost his temper ...
I think he went on foot with her body into the surrounding area and buried her somewhere. Or used his car and drove along some back roads, but not far.