Previously Holden stated he was a "pacifist". Does his description of how he would deal with the "glove thief" support this, or is he just "yellow"?

Holden does lie a lot. He is not the most reliable narrator. The term "yellow" is a figurative word that really depends on how Holden is feeling at the time. Holden's statements of values and opinions cannot be taken at face value: the boy is often a neurotic mess.