He was jealous of Curly. He was upset Anya told Curly about what happened. He was upset he was 'careless' with Anya, and now she was pregnant. He couldn't handle it, he didn't wanna take responsibility.

Locking Captain Curly in the control room was easy, and it wasn't hard to fight the man away since Jimmy knew Curly wouldn't wanna accidentally hurt him. Now he had Curly backed against a wall, waving the ships safety revolver around as he monologed; the ships controls broken and headed straight for an asteroid in barely five minutes. Jimmy grinned as he stared at the captain. The co-pilot was losing his mind.

“I didn't plan this, I swear it, it just... has to happen, you know Cap'n? Who could've thought. This is not my responsibility!”