Definify.com

Webster 1828 Edition

adv.

In violation of allegiance and trust; treacherously; perfidiously.

They had traitorously endeavored to subvert the fundamental laws.

traitorously ‎(comparative more traitorously, superlative most traitorously)

In a traitorous manner; treacherously.
The vizier traitorously revealed the location of the hiding princess in order to save himself.