It all depends. Yep I know it's not a real answer but the situation is simply too general. Several have already assumed that the act is something negative. Why not something positive?
My own personal code of ethics would not allow a hurtful act like a severe assault or illegal homicide without significant justification like defense.
On the other hand doing something nice like fixing a broken bike or something like that when no one knows who did it is certainly something I have done in the past.
I guess what I am getting around to is this. Zeta, refine you poll question a bit further would you. The answers then would be far more revealing as to the personality of some of the posters online "persona" excepting of course those who are incapable of being themselves.