Bears were killed quite effectively prior to the universal availability of premium bullets. When any suitable game bullet impacts a bear's brain cavity at near the muzzle velocity, whatever that happens to be, the cranium being a liquid filled closed vessel, will suffer non-survivable damage, death will come in short order, but more importantly, the bear will stop instantly. That's how its supposed to work in a defensive shooting scenario that takes place at blood on the shoes ranges, making bullet construction almost irrelevant. The problem faced by the shooter is the necessity of making a precise, quick, accurate shot, on a moving target, that is only the width of the bear's snout, hidden within it's massive head.
Then there are additional difficulties which must be overcome if the shot is to succeed, not the least of which includes fear, a target that is in motion, and the angle of elevation if the ground is steep. Fear interferes with target acquisition, with a solid hold on the target, with trigger control, and it tends to compress perceived distance and accelerate perceived time. Target motion presents itself laterally as the bear swings his head from side to side, and horizontally as his head moves with each step, then perhaps dramatically if he steps into an unexpected depression. Shot angle or more correctly shot elevation presents problems related to sight picture if the bear is above or below you. If the range is close, there is sufficient offset between the sights and the point of impact that allowances must be made for the difference, meanwhile the clock is ticking and things are happening.
Its because things don't always occur the way we might expect that I appreciate bullets that penetrate deeply. In the context of defensive shooting, deep, straight line penetration trumps all other considerations, be it accuracy, rate of expansion, or cost, provided that penetration doesn't pose a danger to others. The bullet doesn't break what it doesn't reach, and if the head shot is messed up, or rejected, that bullet had better be able to break and displace bone, and tear up lots of soft tissue, and exit if possible.