Two factors allowed Bill Compton to finally return to Bon Temps. The first was the Great Revelation, which let the world know that vampires were amongst them. The second was the death of the last Compton.
Bill took advantage of brand new legislation to reclaim the rights to his childhood home.
He visited Merlotte's as a way to integrate himself back into the community. Those around him were the great-great grandchildren of people he'd once known.
Amongst them was Merlotte's waitress Sookie Stackhouse. It was practically love at first sight for them (both on screen and off). Sookie helpfully arranged for service workers to repair his home during daylight hours, while at night the couple grew ever closer.
But the course of true love has never run smoothly, particularly when a cross-species relationship is involved.
Everyone from Sookie's human friends and family, through to vampire officials (we're looking at you, Eric), and even deities like Lilith seem intent upon destroying this little romance.
Only time will tell if they succeed.