@degenerateafro
If it is future him, that brings up even more questions. For example, why would he kill the necromancer that helped him out?
If I had to bet, I'd say it was because in the unforeseeable future, being associated with leads to something not in his best interests. It's like...
Let's compare the now-deceased necromancer to baby Hitler, and the audience (us) to baby Hitler's mother. Say someone kills baby Hitler. Our point of view is currently too narrow-minded, too small to understand on a larger scale what is happening or why it is happening. We'll never understand why baby Hitler had to die; in our eyes, a perfectly innocent baby was killed. He hadn't yet grown up into Hitler for us to know. If the murderer was a time-traveler, then they'd know it was for the greater good.
But maybe we'll find out as the MC does more time travelling. The [Necromancer's Lover] skill may have been too strong on her. She reacted very differently compared to any "necromancers" we'd had up til now. It could have lead to her locking him up, like a princess in a secure tower, doing the things she(he) wishes, but at the cost of all freedom.
Edit: Maybe the future self is prepping his past self for the scenario he didn't unlock?