Another possible reason Snape killed DD

[Florence]

No: HPFGUIDX 135287

Cool idea that drinking the potion put the Horcrux into Dumbledore! 
That would give his death more use.  But I agree with dan that
deliberately drinking it just to get rid of one horcrux seems too high
a price to pay.

I like Bob Alberti's theory in 134953 that DD's death put a protective
charm on those he deliberately died to save.  That, or moving Snape
far into LV's good graces (or both), could be worth more than DD's
life and give the Order a better chance of defeating LV.

-oiboyz