5. Why Rick HAS To Die
Firstly, there's the question of whether or not Rick will die at all. The Walking Dead (comic) Twitter account seemed to play down rumours about his demise, and Andrew Lincoln himself said at SDCC that he'd prefer something more like Lennie James' exit.
You can see why that might be appealing, but for Rick's exit to work, it needs to be definitive. He's too big a presence on the series, too all-important to absolutely every facet of the show, to simply have him wander off into the ether and his fate be left open-ended. No matter how long the show ran for, the questions of his whereabouts, of what happened to him, and of whether he might return would constantly loom over the series.
While how TWD copes without Rick is unclear, killing him off at least makes it a clean break, and it's an endpoint Rick is deserving of. His exit shouldn't just be another means of tricking viewers or leaving them hanging on, but a truly huge moment that brings the character's arc to a fitting conclusion.