When you say someone is a so-called pacifist, you are suggesting that they are a pacifist in name only. While they are busy making everyone talk about how peaceful they are, chances are they're also bombing villages to dust.
Using the word so-called is a great way to point out systemic hypocrisy and is a much-beloved tool of street preachers and conspiracy cranks the world over. They might talk about our so-called democracy being in fact a sham, and our so-called democratic elections being rigged by forces beyond our control.