What are arguments for Steve not being guilty?

The stakes are high for Steve: 25 years to life in prison, if he's found guilty of felony murder. The audience knows more than the prosecution does in Monster, though, and I'm going to address your question from the personal angle, not the practical one (e.g., that Steve was just the lookout, or that he didn't get a cut of the money).
First, as readers, we know that Steve is a fundamentally decent person. He loves his little brother, for example. Mr. Sawicki thinks he's smart, talented, and compassionate. Steve even writes in his own journal that he's sure he's not a bad person——although he's trying to convince himself of that fact, too.
Second, Steve isn't violent like the other men in prison, who spout off constantly about hurting others and will pummel a person just for looking at somebody the wrong way. Steve's not interested in that. He thinks his cellmates are nuts, frankly, and all he wants is to avoid conflict as much as humanly possible. He never even considers being violent. Prison is downright terrifying to him, and these aren't his peers.
In the end, Steve is a kid who's made some bad decisions and associated with some disreputable people, but that doesn't necessarily make him a bad person. That doesn't mean he won't keep questioning himself—and filming himself—as he struggles to understand himself, either.