When has Clare ever done things the easy way? She's always shown some level of compassion and humanity when dealing with non Yoma (humans and Claymores).

Ophelia seemed genuinely shocked that she had awakened and everything that came out of her mouth hinted that she was fighting the urges imposed on her by awakening ("I want to east something soft and warm, like guts" and "They don't have to be human guts, but the closer the better") stuff like that made it understandable that she retained some of her senses, unlike the only other pre and post awakened person we've seen who was completely consumed by her Yoma side when she awakened.

My speculation based on these two different circumstances is that your state of mind as you awaken has some factor in how much of your human will stays with you across the change. Priscilla awakened while angry, frustrated, and in a fight she thought meant everything to her. Once she'd crossed over she maintained human cunning but became 'evil' in that she just wanted to win the fight and was willing to sink as far into her Yoma nature as she could in order to gain the power to do it. Ophelia awakened while angry, frustrated, injured and thinking about how she missed her brother who had saved her. Once she crossed over her thoughts weren't on something negative like killing an enemy they were on her brother and that may have lead to her humanity being able to resist the urge to sink further into her Yoma nature.

I'd agree that Clare didn't just kill her outright because she felt bad for her and wanted to give her some shred of human dignity in death. While it may be stupid of the warrior scale it was the more human thing to do, and one of the things they've stressed more than anything in this show is that Claymores being more human in the head is a good thing.