Humane way to kill trapped mice?

i'm sick of my house mice problem. when they take poison they often rot in hard to access places so there's stink and flies, traditional traps are difficult to set and i've heard stories of them not necessarily killing cleanly, sticky ones seem very cruel, so i've bought a large 'humane' trap. according to a pest controller i spoke to releasing them into 'the wild' is cruel as it's not their natural habitat and if you release them close to human habitation you just give someone else your problem. so how do i kill any i manage to trap? btw really don't want to handle them!