I'm planning to steal a lady's stuff
What is it that you are planning to steal from her?
I'm planning to steal her wallet. No, it's probably not a very good idea and I think she'll be mad.
You already know you shouldn't do it. Stealing is wrong, and not only will she be mad, you may get in trouble with the law.
She will not be mad just because I took her wallet.
She may not be mad about the wallet itself, but about the contents inside of it. You shouldn't steal things from others. You wouldn't want everything you own to completely disappear.