If someone kills Keichi...
Well, it depends on the world Belldandy and K1 are living in... I mean 'bout that soul, eternal life, things like that... She's a Goddess, so she just has to know such things about her world. And if it is about "Final Meeting and forever together" - then... perhaps she will behave herself like Yoda of StarWars ("Death is a natural part of life, rejoice for those who became the part of the Force, mourn them do not, miss them do not..." - I like those words)...
So, there is a possibility for her to forgive the one who will kill K1 =) He-he, it will be me....=)

if we apply this scenario in the series...

K1 might be ressurected by the relief office because of bell's contract...

i always forgive eventually in big things it takes delay
so in case of world destruction O.o it wont help cuz no delay after
but things as killing me or something like that i dont think so cuz i never
was dead -__-"
i belive in the words
"to mistake is humanly, but to forgive is godly."

unless my forgiveness go to the limit
sure it has limits but till now it never got to that line
witch i wont fogive after it

I'm pretty sure Belldandy's forgiveness is quite limitless, even if she showed a bit of dislike towards some people, I think she would eventually forgive them.

i'm pretty sure and the manga states so too in so many ways Hild and Mara always bug K-1 and her but still she forgive them and i think forgiveness should be limitless to everyone, having such a burden of hate in yourself burns your body with a black fire and it makes you uncomfortable too, does it not?

well hild does say that bell is most like him. (meaning god) so forgiveness is never ending.

While forgiveness is one of Belldandy's most prominant traits, it also appears to be a personality limit (She forgives almost anyone regradless of their actions, with the exception of Urd).

Personally, I forgive only based on the situation. One who forgives too easily is easily manipulated by others, while those who never forgive are simply cold-hearted.