This is an essay on how to develop the bodhi mind.
http://www.ymba.org/BWF/bwf22.htm
Also I have heard you can think of evil people being deluded and
are sure to fall among the evil paths. You can feel compassion and
wish that they may be reborn in happy realms.
From the link above, there is a page about the 8 sufferings. You
may like to have a look.
In terms of method, the progressive stages of Metta meditation
are good for inculcating Compassion.
First, direct loving-kindness to oneself, then to those whom we
care for, extend it to people we are neutral to, and finally to
those to whom we bear grievances or who are "less lovable".
- A simple, clear set of instructions on Metta practice.
http://info.med.yale.edu/psych/3s/metta.html
- Buddhanet's more comprehensive instruction
http://www.buddhanet.net/metta.htm