Learn English – Idiom/Phrase/Proverb to describe a scenario where a person who saved me from a bad habit has now fallen into the the same habit

The word "camp" comes from the Latin word "campus", which means, essentially a "large open field" – however, the Roman campus had military connotations derived from Rome's Campus Martius, or the "Camp of Mars". It was traditionally a military training ground. In the transition to Old English, the word "camp" lost all notion of a "field", and it was used to mean a "contest, battle, fight, or war", though this definition became obsolete by the mid 1500's, and it gained its meaning as, essentially, a place to camp.

By the late nineteenth century, with traces of conflict still remaining, the word began being used to denote a "body of adherents of a doctrine or cause", and that is where the phrase comes from. It is a military-inspired metaphor, indicating the two opposing sides of an issue are in staunch opposition with very little common ground.

To "fall in", or "fall into", is an idiomatic phrase synonymous with "fit in". In this particular case, however, it may be a reference to another military term, "to fall in", meaning "to line up in a row, standing shoulder to shoulder".

bullying or harassment.

From wikipedia entry on bullying:

Behaviors used to assert such domination can include verbal harassment or threat, physical assault or coercion, and such acts may be directed repeatedly towards particular targets