How does this change katniss when she got her first kill?

Katniss realizes after killing the boy (Marvel from District 1) after he had thrown a spear through Rue after she had been caught in his net right in front of Katniss' eyes. After she had killed him with a single arrow guilt fell upon her and she realized she had just killed a boy she didn't even know the name of.

Hope that helps.

P.S. who ever wrote HK for the previous answer, that doesn't answer my question. If you want to answer a question on wiki answers, plese make it true, positive, and no colorful language. Thank you in advance.