I dont think he is necessarily asking god for the strength to kill specifically.. but the ability to do what needs to be done although it may not be morally right.

Say for instance, someone that robs to feed their child may do the same.. its not a matter of right or wrong, it is just that for that moment that person feels like they have something to accomplish one way or another and need to know that god is with them and will be with them regardless.

I'm not deeply religious by any means, this is just my interpretation.