An ethical dilemma

It was very cold today, very cold indeed. At least a centimetre of snow fell, so of course all the trains went on the blink, and I had to walk from Cannon St to the office. On the way through Denmark Street, a homeless guy asked me for change and I said I didn't have any; as it turns out I did, so that went on a bagel for breakfast, for me. So much for middle class guilt.
But since I'm working on philanthropy for my MA dissertation, the following occurred to me:
a) I don't like being asked for money on the street
b) If I give money to tramps, they'll spend it on booze
c) Giving money to tramps will hasten their demise
Thus (d) follows: give money to tramps, they'll drink themselves to death, and you can proceed to work unbothered. And since we might cruelly assume they'll never lift themselves back from the pit they're in, it's hardly a utilitarian cost to bring about their deaths, and as it's what they want, you'll be respecting their autonomy, so Kantians shouldn't get cross at you either.
A rather weak argument, on several grounds. (b) isn't necessarily true, and even if it were, it would take a long time for (c) to reach its fruition, while meanwhile you'd keep getting solicited for donations, which was hardly the avowed point of the exercise. Plus you may not be respecting the autonomy of somebody if you give into a desire they have whilst deranged by cold and booze, and who's to say that once a vagrant, always a vagrant with no more hope.
So I don't think that's a good guide to life, or anything to add to my dissertation. Otherwise, would probably graduate to kicking people to death, or similar.