"So what exactly have you sabotaged other than your self confidence?"

Bumblebee can certainly take a compliment, but it's not like Starscream forgot what he said earlier. The kid had some inner issues -- not that he particularly cared -- but this still could be a possibly temporary ally.

As much as an Autobot could be one to a Decepticon that is. And to be truthful, Starscream could play nice when he wanted to. The problem was, he generally didn't. But as long as this kid wasn't actively trying to get in his way, he'd have no reason to outright kill him.