When they're on the bridge, Harry kills Ron and Hermione with the Elder Wand and makes Horcruxes, deciding to succeed Voldemort when he is reminded of the wand's power. He is horrified with what he has done, as this murderous feeling left him as quickly as it came. It turns out that the bit of of Voldemort's soul corrupted Harry's soul when it was destroyed, a sort of "spirit poison." He tries to conceal this, but he is becoming more and more evil with time; the poison is spreading.
Ginny is pregnant and gives birth to a boy. By the time of the birth, she and everyone else have discovered what Harry is. He's been banished and has begun to amass an army to take down Hogwarts. His son grows up hated and feels duty bound to stop the new "dark lord."