So you hate him because he stole what you consider as your kill? that's rather petty of you and how was he arrogant? he did not make himself out to being better than everyone else, nor did he think himself as being above the other nations of the Alliance, if anything he was the ONLY Alliance leader who did not bend over and let the horde fuck him in the ass like everyone else pretty much did which is one of the reasons why he is currently its High King.
Because he was the only one who was Willing to not let his people or the other Alliance kingdom's people be killed off and lose land little bit by little bit over time.
AND he was the only leader who actually wants to restore Lordaeron to its former Pure state, and back in the hands of the Living Humans of Lordaeron who are still alive, yet scattered throughout the World, but mostly living in Stormwind.
And he did not push over the other leaders, they can leave the Alliance whenever they choose, Varian is their overall leader because Stormwind is currently the most powerful Alliance Nation, it has the largest Military, the most people, and since the Alliance of today is the successor to the old Alliance of Lordaeron which was an Alliance between the Human Kingdoms primarily. it makes sense that Stormwind, being one of its Original founders, and currently the only Human Kingdom still capable of being a world power, would have the most political leverage over the other nations and races in the Alliance.
The other Alliance nations are still their own nations, and they can do as they please, it is not the Horde and Varian is not a Blue Warchief.
Remember one of the main themes of the Alliance is Tradition, so in turn Stormwind gets the most power in the Alliance by being the oldest of the Original Alliance Kingdoms when it was first formed, and by all the other reasons I stated above.