I have always heard it as "his mad mind had trapped him with a pistol"
This makes perfect sense. He had been trapped in madness when he was armed. The pistol is the "dagger of fire and of steel" that he draws out in the next verse. If he had not had a pistol when he was mad, he would not have shot his true love.

If RT wrote kiss-o, then fine, that is what I will sing. But it doesn't make sense, Sandy definitely sings something beginning with p, and pistol makes sense to me.