I used to have that same question. I still don't have a satisfactory answer, but I have some ideas.
byapada being ill will, but with abyapada one still has enough sila and retraint to block it from forming into a verbal or bodily action.
vihimsa being harmfulness, one is willing to manifest the ill will into a harmful verbal or bodily action.

a related question i have regards how directly samma sankappa is linked  to 3 of the 4 brahmaviharas. I've read in at least 2 or thanissaro's books that he thinks those brahmaviharas are forms of samma sankappa. I don' recall reading any pali suttas where this link is explicitly made. anyone know for sure?



On Sun, Sep 21, 2014 at 6:09 PM, Kumara Bhikkhu kumara.bhikkhu@... [Pali] <Pali@yahoogroups.com> wrote:
 

In samma·sankappa, we find abyapada·sankappa and
avihimsa·sankappa (besides nekkhamma·sankappa). I
still can't see why is there a need to include
avihimsa·sankappa. Shouldn't abyapada·sankappa cover it?

OR perhaps I'm not understanding these terms correctly.

Please enlighten.

mettâ,
kb