Columbine anniversary

what could have made someone careless about murdering 12 people and hurting 24 others?how do you even get that point that you feel like you have to hurt people?did nobody see the signs that they would hurt someone?you really have to think is what i'm doing gunna drive someone over the edge?of couse your gunna think what you do and say is harmless and just for fun but is it really always that way?the boys who did this is had to have a event that changed them for them to react in this way.were they at a fault alone or were the people who bullied them at a fault as well?