
In Massachusetts, you will likely be arrested for murder, but
if you convince the cops/DA that you used lethal force because
of threat of death or serious bodily harm, then the charges
would probably be dropped.  If you run away and are later caught,
then you will have a much harder time convincing cops/judge/jury
of your innocence.  Going "on the lam" is seen as an indication
of guilt by a lot of people.
