'Loaded for bear' is a common American idiom meaning that someone is serious, ready to get a big job done, and armed for the task. If referring to interpersonal conflict, it may also mean that a person is angry and probably ready for a fight. It is very much akin to either cooking with gas, pull out the big guns, or spoiling for a fight, depending on context.
The phrase appeared sometime in the mid-1800s, referring to the larger amount of gunpowder and lead shot (and possibly bigger gun) that a hunter would use when hunting larger game. This person is ready to take down a serious challenger; the violent overtones are present, ready to be subtext if the situation requires it.