Weird civilians who have the right to use potentially lethal violence against other civilians.
Only in the case of imminent threat to life - just like you or I can kill someone if they pose an immediate threat to your life or the life of another.
They're better trained and equipped, but the law of justifiable homicide is broadly similar. Use of lethal force can result in murder charges until they demonstrate that it was necessary and proportionate.
(Except in the US where they will close ranks and protect their own, even when footage shows the use of force to be grossly disproportionate)