Nick love Izzy, she may or may not notice it but he does. He tries his best not to kill creatures like he was trained to do but it was kind of hard when he fell in love with one. Nick didn't feel guilty about being in love with Izzy, he was a hunter and no matter what anyone said she would never be in danger with him there, only his best friend knows the truth and refuses to say anything afraid that she would lose her best friend.