Some people think that John wouldn't have killed Grayza viciously if given the chance there on Arnessk. BULLSHIT! Let me say that one more time...BULL!SHIT!
John is a control freak. Big time. It would have just eaten him up losing control to Grayza like that. Add to that all the shit that has happened to him over the previous three years, and you have one seriously fucked up dude. If he had not used the lakka to stop the effects of the heppel oil. Once she was tied up. I think he would have fucked her up slowly. Look at how he tried to kill Scorpius in LATP...Dipping the fuckers head in an acid bath. If the neural chip hadn't stopped him, John would have. You could see how hard he was straining to do it by the expression in his face.
Screw the fact that she's a woman, over time John has obviously come to think of woman in the PKs differently than those here on Earth because of Aeryn. But Grayza cheated, she used the oil to gain control over John...making him fuck her and tell everything to her. We don't know everything he told her because we were only showed a portion of this. But I'm betting he told her shit that he would have fought Scorpy tooth and nail to protect. Look at how easily he described wormholes to her at the simple prompting of a question.
But John is a hero...and people don't like to see their heros shoving a knife inside a person, much less a woman, and just leaving the knife there. They seem to forget that John is Human...fallible...emotional...and given to bouts of acting on irrational compulsion because of these factors.