Honestly, I don't think Shaka backstabs people. There definitely are leaders in the game who are prone to do it, but I really do not think Shaka is one. He's never backstabbed me, unlike Ragner and Montezuma. Shaka seems rather loyal if you're good to him. Of course, if you don't give into his demands and don't build many military units, he'll destroy you, but that's expected...
Yes, I don't think he actually considers it backstabbing... It's more like everyone he meets is potentially a part of Great Zululand to him, it's only a matter of time. If being allied and nice gets him there faster, so be it.
Shaka is scary, I always know it will be hard time when I see him. Though Gilgamesh is quickly climbing my "Most Hated" list.