Strings (2004)

The previous summary has it backward. The king commits suicide out of guilt, and tells his son to make peace with the enemy. But his evil brother destroys the suicide note and makes it look like he was killed by the Zeriths, to persuade the prince to go out and avenge his father. His plan is that the prince will be killed and he will take the throne. The prince heads off, with his advisor/vizier, but the evil brother blackmails the advisor to get him to kill the prince before he even finds the Zeriths, by saying he will kill the advisor's family. The advisor's wife finds a copy of the suicide note (the page that was underneath when the king was writing the letter), but the evil brother has a plan to take care of that. I'll say no more, because spoilers.