Honestly, on a pvp server, it happens. You do it, they do it and everyone has justifications for it. Personally, I will gank for several reasons.
#1 You look like Alliance!
#2 It would be a challenge
#3 I have been ganked too much today and someone has to die!
#4 Someone from your guild has ganked me before! Now YOU have to die.
See how many justifications you can come up with!
These are 2 of my "I am lv 70 and I can gank you!" people. I was farming in Nagrand when I got hit up with these guys. I killed the Druid and a Windroc >.<>.<>
The second one is a Rogue with an annoying macro that "shoo"s you. I have to give him points tho... it aggravated. Wonder if he realized I was aggravated when his lv 70 butt was on that cold cave floor and he was corpse running?
Also, I got tagged by a lv 70 Ice mage... you know how that one ended and I am NOT posting that shot lol.