Wanted to protect? He was well past protecting, after having saved Armin. He was in the titan's gastric juices already, wondering if that's how he's going to die, without achieving anything at all. If he had a mental image, it was that: Why he's about to perish before having vanquished scores of titans like he swore he would. Now his mental image should be no less: How to move the big rock.
Edit: Uh... I completely managed to forget the cannonball transformation. So, yeah.