I wouldn't attempt it for a number of reasons.
1. I would be doubtful if I could do it. If I failed to kill the horse in twenty minutes, I would simply be visiting mindless violence on a defenceless animal for no good reason.
2. I would be doubtful that anyone could guarantee that ALL Political prisoners would be released, given the broad definition of the term, the political ideologies of those that incarcerate people for political reasons, the sheer scale of the task. If (assuming I wanted all political prisoners released unconditionally) I could not be guaranteed that this would happen, again I'd be doing violence to a defenceless animal for no good reason
3. I may not want all political prisoners released. Sure, the vast majority of Political prisoners may have been detained for protesting against injustice, or have been locked up for no other reason than questioning their government, but if there were any that, upon getting further details, may be classified as a "political prisoner' (as defined by Amnesty International), but who I believed deserved to be behind bars.
Bottom line, I find cruelty to animals abhorrent! I do believe in the concept of the greater good (sacrifice the few to save the many), but you really need to be sure that you'll get the desired result (i.e. after sacrificing the few, the Many are not saved)
If all in this theoretical situation was acheivable, I'd put the boots on, Kick the horse to death in under 20 minutes, hating every second of it, rejoice with the freed prisoners, and spend the rest of my life hunting down and kicking to death (much more slowly this time) the sadistic K#%&tz that put me in that situation
That was cheery for a Thursday morning!