I have to agree with the drowning thing. That's about as farm from humane as you can get.
I never had a true experience like mattblackburn, but I went through aircrew school and know that water + long time = super fear. Why else does water-board torture work so well?
Humane can be messy, the only important part is that it is quick. So get a hammer, pellet gun, baseball bat, whatever can get them out in one quick smash.
I hate rats and to be honest fully support some not so humane deaths. Here in China they have a phrase: "Kill the chicken in front of the monkey." Bad phrase for a chicken lover, but good idea. If you want someone gone then show them what’s in store if they stick around, prefably on something that makes a lot of noise before it's dispatched.
A few rats stuck to a trap screaming will certainly give the rest of the buggers a chance to think about whether or not they want back into your feed bin.
Living in the city I wholly support glue traps. They work well, will continue to work for months even in a damp environment and don’t allow a poisoned rat to wiggle into someplace that you can’t find, stinking up the area for months. Poor neighbor.
Also some poisions are active long after death, good idea to be worried about pets.
We had a poisoned rat silk into our window after we cleaned any leftover rats the previous owner left behind. We wern't looking for them anymore and let me tell you even though we caught it right when the stink came it was cleaning for day’s straight to get the stink out. DAYS of terrible stench and after 3 months you could still smell it near the floorboards. Even now when I walk in the room my memory provides me with a quick refresher... terrible.
Unfortunately nature makes rats an impressive pest and any remorse will give them a huge edge they will exploit making the problem even bigger.
Glue trap every hole you find, they will eventually think its safe, or just be curious enough to walk on them and trap themselves. To be honest a glue trap and drowning aren’t much different, it’s still a struggling suffering death. To me the glue trap is better, at least the rat can convince itself its making progress for any pain endured, while drowning is 100% suffering.