Some people would say that murder is the worst crime, but I disagree. A murder done in anger, or to get revenge, or for money, or is otherwise a personal thing is one thing. But there are crimes against society - like the Boston bombers, and the school shooters, and the people who deliberately drive into crowds, or poison the water supply. Those disrupt society, and the cost of them goes far beyond the people killed.

Those criminals need to be in jail for life.

Mass murder being worse than a single murder seems self-evident. I don't want people who have done either to be roaming free in society. Granted a killing may be a crime of rage or passion and not methodically planned, but generally that would be charged as manslaughter and so the difference in motivation and effect on society is already recognized.