And to think that you're even a girl? OMG! D mada!
Anyway, I think mentally disturbed ang usual reason anang ing ana na killings. Cause could be illegal drugs and/or alcohol, childhood trauma relating to voilence, childhood experience of abuse, nervous breakdown, etc. The list goes on.
There is also one reason that an ordinary person with no history of violence can kill. That is extreme threat to self or the person's loved ones. It can be a defense mechanism too. Especially, when emotions run high. Sometimes, emotions cab be very very hard to control.