It's funny how people forget all the heinous shit Genn did before Cataclysm and Legion made him out to be some kind of hero.
Refused to actually support the Alliance of Lordaeron, only sending a small token force of troops to help fight the orcs during the Second War, then seceded from the Alliance because he didn't like paying for the internment camps, which caused a civil war to erupt in Gilneas between Genn and the royals and Crowley and his Alliance loyalist rebels. Withdrew behind the walls of Gilneas while the Alliance of Lordaeron suffered their fate at the hands of the Scourge. Ordered Arugal to summon the worgen to hold the Scourge at bay, causing the outbreak of the worgen curse in Gilneas.
The Forsaken (aka undead Lordaeronians) have every reason to resent and hate the Gilneans for abandoning Lordaeron to hide behind their walls. They were haughty seclusionist cowards as far as anyone in the Alliance of Lordaeron are concerned. Varian Wrynn even spat in Genn's eye when he showed up in Stormwind begging to join the Alliance.
And frankly, if not for some night elf meditation magic bullshit, the worgen would be no better than the fel orcs: bloodthirsty monsters with cursed blood pulsing through their veins, some even volunteering for it.
It's all incredibly hypocritical.