I disagree there is anything wrong with your opponent's actions.
When you begin a game, you know the rules beforehand (this goes for any competitive activity). That includes knowing you have only so much time in which to make your moves. If you fall behind in time to your opponent, or if your opponent makes faster moves or plays a "better" game, why should s/he not try and win? Is not the object of the game to win?
I've won many games because of time while at a material and/or strategic disadvantage, and I have lost many games on time while ahead of my opponent. Either way, I knew the rules of each game when I entered them and I accepted the outcomes regardless of my position, win or lose.
I understand your level of frustration. There are many times I wish the game had another 60 seconds because it's gotten so good and I wanted to know where it would go, but it is what it is. The game has rules and definitions, and we play each game by those.
The NFL uses clock mangement all the time to win games and fans expect their team to win. Yes I realize that this is not football. People keep calling chess a sport and a big part of many sports is time. Accept your loss with dignity.