I'm not helpless, Lucifer. I'm stronger than you think. Istredd says, more sharp than usual. Just as he snapped at Geralt before for underestimating him, he does it again. He is not devoid of his own triggers. After a long time of fellow mages looking down on him for being more interested in books than power, he has just enough of a sliver of pride that can get bruised. His frustration gets through with a prickle of defensiveness.

You're my friend, I'm not going to stand by. It is slightly hypocritical to say that considering he just went somewhere Lucifer couldn't follow and made him fear for Istredd's life for a time. He forced him to stand by and let it happen in that case. He's caught, even he can be a hypocrite.