I doubt she convinced him it was morally wrong. More likely she convinced him that leaving him alive might be useful.
It's not so much about defending Orstead as it is finding you guys' outrage to be really weird. A mysterious character showed up and tried to kill our heroes. Like, that's pretty common, but you guys are really pissed off at this guy for some reason.
Honestly, it kinda feels like we got some manga prophets who know more reasons to hate this guy...
If anything, I'm more pissed at Rudy, for ignoring Rujierd's "Do't do anything!" and almost getting everyone killed.