Remember that he isn't "changed" until the crucible is actually hooked up to him, so up until get into the citadel(unnoticed by harbinger who is probably nearsighted or something) and hackett receives a convenient text message from faceless soldier mook B, the catalyst is still in complete control. He could've easily turned off the citadel beam, whathave you.pilouuuu said:*snip*
He fits the definition of deus ex machina to a "T". The term comes from greek plays in which a conflict of epic proportion would be neatly and conveniently solved by a god "deus" literally descending from on high via a pulley machine "machina" to wrap everything up. That is EXACTLY what the star child does. He can conveniently use the crucible to send out a giant "FUCK YALL" wave of destruction across the galaxy to fix everything, despite such functionality never being alluded to before.
He also fits the term literally as well, he is a "god" (of the reapers) that comes from a "machine" (the citadel).
Despite any explanation his existence still renders the entire conflict of Mass Effect 1 pointless by nullifying any purpose of the keepers, and renders the otherwise excellently characterized harbinger(up until he got no lines in the 3rd installment) redundant and pointless as well. Not to mention if he's the citadel he could easily at any point during the trilogy just have turned off all artificial environments, killing the entirety of his enemies' leadership.