They're definitely setting this up for a long and really dark storyline. I don't think they'll have him kill again until he's a bit older but it will happen eventually - the looks and comments he's making are not normal. And of course Ian and Jane ignore it and instead of getting the hell child the psychiatric help he needs, he just gets worse and worse.
I remember when the 'we need to talk about Kevin' reference was made that a lot of people were trying to dismiss it, but it's pretty clear that they are going down that route with Bobby.