Meaning of the terms used

Only Full-Fledged Angels were allowed to venture the human realm. In-Training Angels were forbidden roam the land, for they were not ready to meet the dangers. However, an In-Training Angel by the name of (Y/n) rebelled from this rule, and from that lead her to meet a demon, Thomas.
Even though their races are at each other's throats, and once in each others' sight are said to kill on the spot. Thomas didn't instinctively kill (Y/n) during their encounter. Instead, Thomas had different intentions for (Y/n), which was to be his mate ( much to (Y/n)'s dismayed).
But why?

!!NOTE!!
I DO NOT OWN THE VIDEOS USED, THE BASE OF THIS PHOTO, NOR HAMILTON THE MUSICAL. ALL RIGHTS GOES TO THE ORIGINAL CREATOR!