I think in the US white killers tend not to be labelled terrorists, partly because they are often not as ideologically driven, but also there is a kind of not subtle racism at work. Brown people are terrorists, and white people are just mass killers, then we know who the real enemy is, and we can ignore the access to guns. But Sandy Hook has been called domestic terrorism, although the motives are unknown. (But quite a lot of white killers seem to have an anti-government agenda).
But there is a push to use the label 'white terrorist', for example, the killings recently at the abortion centre (Planned Parenthood).