Just to make it clear, Yes, Jace just went on his own to find Valentine. And that noise you hear is me headdesking myself into oblivion. I also to have now ask: Why the fuck is he going to do once Jace found Valentine? Jace had an opportunity to kill him at the end of ‘City Of Bones’ and he couldn’t. So what is he going to do now?
What also makes this decision of Jace momentarily idiotic is that Valentine can quite easily use Jace as a leverage against Clary. Valentine can hold him for ransom and because Clary is stupider than a bag of rocks would run impulsively to save him.
So in the end, this confrontation with Valentine makes zero logical sense and it feels like it was just pulled straight out of her posterior.