Yeah, I'm still going through "Anomaly" and am overdue in writing up a review of it. Archer's transformation really starts here I think, when he shoves that guy into an airlock to force information out of him. Good natured Archer of seasons 1 & 2 would have never considered that.
Yes; this is the turning point. Other things are done later (e. g. a theft), but this is the moment when innocence is first truly lost.