At cat size, the bee wouldn't have the problem of its stinger getting lodged in human skin and being unable to safely free itself, but if we're envisioning a bee that just naturally wound up that big I would honestly think its stinger would end up being vestigial since it could just bite the kinds of things its stinger is for and anything bigger would have to contend with swarms of cat-sized bees doing the thing small birds do to chase off predators where they just make it fuck off by buzzing it repeatedly. A dozen of these giant bees dive bombing and circling around would probably be enough to drive off even a bear.