Two things: Major depressive disorder, commonly known as depression, is a mental health disorder rather than a disease. With that said distinguishing between the two is splitting hairs on gamefaqs. External factors can trigger depressive episodes, but for those with the disorder, depression can appear unexpectedly, much like rain on an otherwise sunny day. It is not something that can be controlled without proper help and treatment.Managing this condition often requires professional help, as it is not something one can simply overcome on their own. So there is that and some people are seemingly over looking this.
However with that said, having this disorder does not excuse harmful behavior towards oneself or others. Even when experiencing a depressive state, individuals are responsible for their actions. Choices made during these times are still personal decisions. Therefore, if someone harms another person while depressed, they are still accountable for that choice.