The human mind coupled with a mental illness like Depression can have a strong influence on a persons actions. Add to this Bullying both IRL and online before long the illness gets the better of most people, after all they are human and have feelings. Unfortunately some people can not control their impulses, she states so in the video, no matter how hard they try. People dismiss this as adolescent drama, attention seeking, etc when it is in fact a illness that requires some form of treatment or the result can be what happened in her case.
BTW: This is not isolate to teens but it is more publicized when a teen commits suicide. Adults do this sort of thing also.
Stop trying to reason why she did it because none of us really knew her, we just know of her, so her real reasoning for taking her life will never be fully clear. We can not help her any longer, but we should be asking how can we help prevent the next Jade/Joe from ending their life.