Terror Attack in Britain

Several issues here. the WaPo reports that the man responsible was Brih born, although Muslim and influenced by terrorism (along with a criminal record). And of course Donald Trump Jr. attacked the mayor of London by taking his statement about preparedness out of context (WaPo). What this tells me is that nativism will not protect us; terrorism is more and more likely to be homegrown. And also, what if Londoners had guns? Would it have been worse (because he would have a gun) or better because others would have guns?