Can someone stay happy after ruining someone else life?

That has nothing to do with karma. As for staying happy after ruining someone's life, please explain. You may not have ruined anyone's life and not understanding the situation may be confusing you.

Karma usually happens more when something becomes a lifestyle. One mistake generally doesn't 'bring karma look at things more as what am I meant to learn from this.

Sure they can if they are heartless.

I think Karma, is the same as consequence.

If you do something to harm another, on purpose, eventually, Karma, or consequence, will happen because of what you did.

Guilt sticks around until things have been taken care of, i.e. an apology or confession. It's really honesty that can set things right, as hard as it may be.

Sorry, every religion does not believe in karma. Another one of your incorrect blanket statements.

Does your signature refer to the Gülen Hizmet Movement?