The reason all those forms are acceptable is because, based on linguistic theory, that spot after the comma can be filled by a complement or a referent, the complement being "that" and the referent being "it". Both are transitional, but the "it" version is redundant in the information it effectively relates, that being some situation X where person B was mean to person A at some time where tpresent with the condition of meanness having being asserted)...if that makes any sense :P. Thusly, that is somewhat preferable in simply acting as a segue into the next action while holding onto the information previously stated.

Nevertheless, redundancy occurs throughout language in many facets (i.e. adverbs differing from adjectives only in what they modify and an -ly ending, the meaning of the adjective base is the same, the -ly simply shows acknowledgement that that adjective (modifier) is modifying a verb instead of a noun