Well that is much to general...I mean....one murder is very different from another...now isn't it? So let us ask in a more precise manner.
What if the killer thought they were doing it for a noble reason. A way to make the world a better place? This would be common in war...I would assume that both sides...each soldier...doesn't feel much remorse because they are fighting for a cause. Right? But then again I haven't been to war. I feel for our men that fight for our freedom...but I also feel for the other side...surely they believed in something.
So...not to get all political...I have a character. She killed some people...but she really believed she did it for a higher purpose. So would she feel regret?
I have other characters that kill in cold blood...and no they don't feel remorse, not really. But this character....she believes in creating a "perfect" world...in fighting for peace...and love...and the light...but she killed...So I think: She has to feel some kind of regret...right?
What do you think?
P.S.: I hope you are having a wonderful day!!