In the present context, the term outside
contrasts with those who entered the city and so are now inside.
Those outside
did not or could not enter because they did not wash their robes. So they also do not have access to the tree of life—which in turn means that they remain in their death (as wages of sin, Genesis 2:17). By remaining outside
they also cannot contaminate (the life of) those who are inside (Revelation 21:27).
15 Outside are the dogs and sorcerers and the sexually immoral and murderers and idolaters, and everyone who loves and practices falsehood.