Armed police terrify me in any airport, or indeed in any crowded situation...for the same reason. If they actually use their weapons they'll cause a massacre of the innocents, not just one Brazilian.
The only time I have seen an armed attack on an aircraft at an airport (in the Middle East) the numerous armed police slowly and carefully put their weapons down, thus saving the lives of several hundred people. Would the UK Police do the same? I wonder.
All the UK Border desks at LTN were manned today at the time the problems I described happened; there simply are far too few of them. That's the airport's fault, not UKBA's.