Board Thread:Spoilers!/@comment-24674051-20160927161944/@comment-25375217-20160928024211

To be fair, taking that freaking serum seemed like a reasonable thing to do and wasn't caused directly by Robin's death. It was several events that happened over the course of Seasons 4 and 5 - Robin's death was just the straw that broke the camel's back.

However, every death prior to Robin caused Regina herself to do bad things or was caused by Regina doing bad things, not some strange "dark side of herself" (which in theory Snow would have too). Daniel dies? She holds a grudge against a ten-year-old that leads her to murder innocent villagers. Henry Sr. dies? So that she can cast the Dark Curse to get revenge on aforementioned ten-year-old. Cora dies? She tries to kill her son's entire family and force him to love her. The fact that she hasn't purposefully done anything to harm someone shows a lot of character growth already - the worst she has done is blaming Zelena, but she even tried talking that out.