She held back at the end when she literally had her fist inches away from his face.
The fact that Mira did not kill him means Mirajane doesn't just go around killing people, no matter how pissed she is.
She also didn't kill him because as per her words, they're "comrades", "guildmates", they "laugh together, carry on together" and they "walk together".
It's a power of feelings speech.
Seeing an image of her sister get 'erased' once again? No it wasn't, she was genuinely pissed.
There's another one in chapter 36.
There's two scans from two different chapters with the word of god saying that she was enraged, POF; not holding back.