Can a bear kill a bull?

That depends on many factors: age and health of the lion, species age and health of the bear, etc. In reality, size and weight really doesn't have much to do with whether a bear could kill a lion or not: it actually has more to do with experience and fighting-ability. A young bear would be foolish to mess with an adult lion, whereas a young lion would be foolish to mess with a larger bear like a grizzly, brown or polar bear, or even the more aggressive black bear.

Between an experienced lion and a male black bear, it could be a draw. Both are aggressive as the other, even though one may be a little larger than the other or the same size (most black bear boars are larger than an adult lion), because both have to defend their territories from rivals.

With a lion versus an Alaskan brown bear, a Grizzly or a Polar bear though, it's likely that such a bear could do enough damage to a lion to kill it. This scenario is most likely with a polar bear vs. a lion than a grizzly/brown bear vs. lion because polar bears are adapted to hunt and kill animals. But so are grizzlies and brown bears, since it's not uncommon for them to hunt down and kill moose, deer, or caribou if they're hungry enough.

So to answer the question, there really is no definitive answer because of the reasons stated above.