Subversive

[Sarah]

Subversive politics: Subversion refers to an attempt to overthrow structures of authority, including the state. It is an overturning or uprooting. The word is present in all languages of Latin origin, originally applying to such diverse events as the military defeat of a city.
Subversive activity: Subversive activity is the lending of aid, comfort, and moral support to individuals, groups, or organizations that advocate the overthrow of incumbent governments by force and violence. All willful acts that are intended to be detrimental to the best interests of the government and that do not fall into the categories of treason, sedition, sabotage, or espionage are placed in the category of subversive activity.
And last but not least:
Subversive behavior: Subversive behavior is to undermine somebody with various trick (lies, cheats, backstabbing, false advice etc...) where the intent or goal is not visible at all.

[Dave Paprocki]

Anything that is [figuratively] and/or literally [harmful] and [destructive].

[Dianne]

Seeking or intended to subvert an established system or institution.

[Monty]

trying to destroy or damage something, especially an established political system: