Game is made like that, boths sides have scums, I play horde/alliance and I see those kind of people on both sides....
I'll write you some reasons why are they doing it:
1. Bored players, they want to kill lowbies for fun to make you mad.
2. Bored donors with full BiS gear, mostly they are ganking lowbies since they cant win in arenas or 1v1 against their level.
3. People who want to take revenge, they was also killed when they was lowbies, now they want to revenge.
4. Horde/Alliace haters, enemy is enemy, no matter if you're low or high level, you're still an enemy and they look it that way.
Me, as a donor, I love to kill lowbies and make them mad, when I see that they login on other char and insult me then it makes me happy and laugh, because I know how mad they are and I dont give a fck about them...
This game can make you relax, makes you mad, makes you happy, always depends, try to relax my friend and accept the situation how it is, you can't change anything.