Not necessarily.

It could be racism, in the case that the cleaners are of an ethnicity the attorney despises.

It could be bullying, in the case that the people whose change he is stealing deliberately leave cash in their desks for paying vending machines, or because they know they are prone to leaving their wallets at home.

It could be that he was so well off that he couldn't see the problem with taking a few dollars from a desk to pay a parking meter or whatever; then became embarrassed when confronted.

Feelings of wealth vs poverty are not absolute. If the attorney has an expensive plastic surgery habit, all his friends are on $500K, and he can only afford two trips to Klosters this year because his yacht needs servicing; the money may have utility to him in an "every little helps" kind of way. He might feel that everyone else is better off than he is.

If this is a big office, you could probably make $100-$200 in a session of theft (I know there is normally at least £15 in desks in my office of 8 people). If his salary is paid into a joint account, and he wants to pay for something without the knowledge of his partner (e.g. charlie and hookers), then stealing cash might be the easiest way he can see to conceal this spending.

Just because its a mental disorder doesn't mean the guy isn't being an asshole.

I'm sure every 'asshole' will sooner or later be diagnosed, at least in US. Might as well declassify every diagnosis not related to obvious physical damage back to the level of 'assholes needing cultural education'.