Are you a true maniac?

This is a test.

I don't know the whole story about this test - just some brief details. Researchers created just one question and they claim that only a real true maniac or serial killer could answer it correctly.

Hmm, I'm not sure - it took me 5 minutes to figure out the correct answer... Am I...? So. here it is:

A girl saw a very handsome young man at her mother's funeral and fell in love with him at the first sight. But unfortunately, he walked away before she could even ask his name. Two weeks later she killed her sister. Why did she do it?

Dear friends, please tell me I'm not the only real maniac here 

Try to answer it too