Mass murderers do not usually get out on bond.
But, at least in the US, most people who are not a flight risk are usually given some sort of bail amount. Once someone is arrested they see a judge, who will consider two main factors in determining whether an individual is eligible for release:
Whether he/she is a flight risk; and
Whether he/she is a danger to the community.
Other factors might be:
They are habitual offenders with an extensive criminal record, and have had multiple "failure to appear" problems in the past.
Their crime is especially serious or violent, such as a felony sex offense or retaliatory or gang violence.
Their criminal behavior places others at risk of serious injury or death, such as selling drugs and drunk driving, weapons charges, and something causing harm to children.
They are charged with a capital offense, such as murder of a law enforcement officer.