Considering all that's been written so far, I would have to conclude that the guillotine is the most painless way of executing humans or nonhumans.  I read somewhere that this is how lab mice and rats are killed when the lab people are through with them.

Therefore, if slaughterhouses were eqipped with guillotines of the proper size for cattle and pigs, those animals could theoretically be quickly and painlessly slaughtered.

And, regarding animals being used in religious rites or rituals -

Freedom of religion should not include animal cruelty in any form.  In any case, most of the people who think that it is perfectly permissible to use animals or birds in blood sacrifices or other religious rituals are hypocrites.  

Because doubtless they would be horrified if dog fights were to be made legal because some group said it was part of their ritual.