There are probably always a significant number of people suffering with depression who are already close to committing suicide and it's easy to nudge them over

There are probably always a significant number of people suffering with depression who are already close to committing suicide and it's easy to nudge them over the edge (unfortunate choice of words, but I can't think of another way to phrase it), and I don't think the same is true about people who plan on committing murder. There's usually a specific reason why people commit murder, whereas depression usually happens over time and can't be blamed on a single happening.