I'm thinking of stealing client information from a law firm I worked for
Are you serious? Why would you steal information from your own work?
It's for my own personal use.
It's wrong to steal from your employer. It's also illegal. If you get caught you'll likely get fired or worse, charged for corporate espionage.
I'll say I lost it and won't get caught
Even if you don't get caught, it's wrong to steal from your company.   You should leave these people's private information alone.