Actually, territorial battles are not uncommon, they may just not be as common in Canada as they are in some places in the US. And most of them do not result in death. In recent years in the US Midwest, there have been a number of resident birds, both male and female, lost at nest sites as a result of territorial competition. Some are even found dead in the nestboxes they refused to give up. And yes, it is a tough thing to watch. Nature frequently is.