haven't read the books or anything, just seen the movies
so the ball actually never said anything about how to kill harry... voldemort just thought hearing the rest would be helpful?
In the book it's the prophecy that Voldemort heard that made him make his decision to kill Harry Potter. He never heard the whole thing, so he wanted to hear the rest, thinking it would tell him how to once and for all kill Harry. This propecy is what started everything. It says that Neither Can Live While The Other Survives.
because he had only heard part of it, so he wanted to hear the whole thing. and it also tells him how to finish off harry.
(well, it actually says "The one with the power to vanquish the Dark Lord approaches... Born as the seventh month dies... Born to those who thrice defied him... And the Dark Lord will mark him as his equal.. but he will have power the Dark Lord knows not.. and either must die and the hands of the other for neither can live while the other survives... The one with power the vanquish the Dark Lord will be born as the sevnth month dies")