Use ranged weapons to kill the dire boar from a distance. It will be safer that way.
Attack the dire boar with close-range weapons, such as your spears and sword. It will be more honorable that way.
Gather your things and continue your trek tonight, leaving the boar tethered. Perhaps you or someone else can come back in a day or two to free it, once you've reached Brother Owen.
Try to find some way to free the boar but force it to run off, so it's no longer a threat. That way, you won't have to kill it. (Please comment details).
Some other creative option? (Please comment details.)