Graham died because he strayed off his Storybrooke path and started to think for himself. It didn’t have anything to do with kissing Emma – that was thrown in there by the writers to make us think that kissing Emma was the reason he remembered. Emma coming to town started time moving forward which has caused things to happen which are making the characters have deja vu moments or are making them realize that they are feeling unsatisfied with their life in Storybrooke. Once they start to do something about these things they start to remember more of FTL and/or they get more confident and begin to make their own decisions which causes them to deviate from the set life/path they had in Storybrooke. This unsettles the balance that had been on-going for years which creates a ripple effect and other characters then begin to feel that something is off in their own life and the pattern continues. I believe that Regina is killing people or making them disappear in order to try to maintain or restore balance so that the curse can continue or not be broken by Emma. Graham realized that he didn’t feel anything and then Henry explained that it was because he was the Huntsman and had his heart removed. So Graham realized he was missing his heart. Once his heart was freed from the box in Regina’s vault he could remember his FTL again – but them Regina crushed his heart and he died. I think Regina did something similar to Kathryn because she realized she didn’t really love David – just like Abigail realized she didn’t love James.