During one of the darkest periods of the Chassidic movement—when the opponents of Chassidism had permitted the bloodshed of chassidim—the Alter Rebbe sent his prized pupil, Reb Aaron of Strashela to attempt a mediation between chassidim and their adversaries.[1]
These mitnagdim were G-d fearing and thoughtful individuals whose intent in their opposition was truly for the sake of Heaven.
They asked Reb Aaron:
What is a Chassid? It seems that he is worse than animal, for, when an animal is slaughtered his blood must be covered, however, one is permitted to slaughter a chassid without concern for the prohibition (Vayikra 19:16) “You shall not stand upon your fellow’s blood.”
Reb Aaron responded:
A chassid in an individual with an internal fire. He has a constant thirst for G-dliness which is compared to fire, as in the verse (Devarim 4:24) “For the Lord your God is a consuming fire.”
Although a chassid is externally calm and collected, this is the true identity and content of a chassid.
[1] Sefer HaSichot 5704, Pg. 158