Anger comes out in various ways.
I think you are angry at life. Something obviously has happened in your life to make you feel like this against someone you don't know and the easiest thing to do is take out anger on a stranger. Just by reading what you post, how you post it and what is said, any normal personal can tell that you are full of anger and resentment. For a start she beats your favourite tennis player and I'm sure it doesn't help that some of her fans rub it in your face, which makes you even more angrier.
That's just for starters, but it's obviously not the reason WHY you are angry; you just let it out on a stranger especially when that stranger embarrasses someone you admire
You also think you are right all the time and if someone tries to debate anything with you and they don't agree with you, you insult them - which shows immaturity and anger. You cannot just say what you want to say and leave it at that, you have to keep on and on and on to prove that you are right and know everything. That shows anger right there.
Anyway, the reason why I wrote the above was because you asked "why would I be angry at Serena"? Therefore you wanted an answer, otherwise you wouldn't have asked the question