Michel's character in the beginning is shown in an almost innocent light, even though he's trying to be a thief. Which isn't exactly divulged in clear terms. It might be that he's trying to pay back his mother, or e might be broke and bored. Then he's progressively revealed as being a determined thief, who's looking for ways in which to get better.
It seems that it goes from being a serious hobby to an obsession. Almost as if it's a compulsion, that he can't keep from resisting. Constant urges overwhelm him into doing the wrong thing (as in stealing from his friends jacket pocket).
His character seems to have found a line, then crosses it, and there is no looking back for him. Bresson shows the lack of need, for a complex script and uses images and sounds in order to sell his story.