Warlocks =/= demon hunters. Night elves kill those of their race that try to become warlocks. They however, do NOT kill demon hunters.
At this point in time, Warlocks might as well be Demon Hunters... since they pretty much have nearly every single ability that Demon Hunters used to have. In fact, one of the core people they learned new powers from was Illidan. Their motif is fight fire with fire, which used to be what Demon Hunters were all about, as well.
They even have a glyph called Demon Hunting and access to transmogrifiable Demon Hunter garb.
No.You're wrong. Night elven warlocks aren't shunned. They are [killed. There was a quest in classic Desolace where you had to rescue a female night elf. She had gone to Desolace with her brother to study something there, but he tricked her and ran off to the Burning Blade to learn how to be a warlock. The woman, his sister, was tied up. She gives you a quest to kill him because she doesn't want him to become a warlock. It's seen as the ultimate evil to be. Demon hunters on the other hand are just shunned because of what they do. Night elven society recognizes the sacrifice they make, but are very uncomfortable with it so the demon hunters are loners and roam the outskirts of kaldorei society.
Maybe that Night Elf was executed because he joined the Burning Blade rather than because he became a Warlock? The Burning Blade are considered an evil group, and he wouldn't need to join them at all if he wanted to know how to become a Warlock.
I'm also pretty sure that Varian would back up the admittance of Night Elf Warlocks, provided they are trustworthy and mean to serve the cause of the Alliance... rather than rogue groups that used to serve the Legion.