What is sin?

A deed that is committed intentionally for the purpose of harming and injuring anyone, not only by physically, but also mentally, emotionally and morally.

Who is a sinner?

A sinner is a person who commits such a deed that harms and injures the other person for his own selfish motives.
Do you become a sinner if you commit any bad karma?
Yes, of course, you are. Since, your own sinful intentions only inspire your bad karma.
However, if you commit any bad karma for the purpose of benevolence for the society, then you will remain unaffected by any sin.
For instance, in the great epic Mahabharata, the Pandavas fought against the Kauravas in the battlefield of Kurukshetra only to reestablish the righteousness (Dharma).

How can you overcome from any sin?

You can overcome from any sin if you surrender yourself wholly to Almighty God and realize that everything in this Universe is His creations. He is the proprietor of everything in this world. And when you lead yourself to the righteous path.