Warning: The following contains SPOILERS for The Walking Dead season 11.
Father Gabriel (Seth Gilliam) is one of the most complex characters in The Walking Dead but generally a good man — so why did he lie to Maggie about killing the Reaper? Gabriel promised Maggie that he would kill any Reaper should he have the upper hand, yet when he found himself in such a situation, he chose not to. In a time when his faith is dwindling, why did Gabriel choose mercy?
Even after a very close brush with the Reapers in the safe house, Walking Dead's Maggie is undeterred from her plan to attack Meridian. While on a mission to scout out Meridian ahead of their attack, Maggie made Gabriel promise that he would kill a Reaper if they were alone and Gabriel had the advantage, which he easily agreed to — but when Gabriel came face to face with a Reaper who was praying in the cemetery, he made the decision to spare him. Gabriel had the element of surprise, as well as a bigger weapon, yet he chose to use neither. Upon returning, Gabriel lied to Maggie when he told her he hadn't encountered a Reaper.
It's possible that praying over the graves of the Meridian citizens in The Walking Dead left Gabriel feeling sentimental for the times when he believed what he was saying — seeing someone pray with the fervor he once prayed with seems to have shaken Gabriel's steadfast belief that God has abandoned them. While he is struggling to believe, this man before him has managed to hold onto his faith and if this man maintained his faith, it is possible Gabriel could find his own again. This battle has caused his morals to shift frequently — and it's likely at the root of his actions. Even if Gabriel's faith is faltering, the lessons of the Bible are still ingrained in him, and "thou shalt not kill" is a pretty big one. If there is a chance that God exists, then Gabriel may be judged for his actions against the Reapers one day.
Gabriel is a multi-faceted man. Part of Gabriel is battle-hardened from the trauma he has experienced in the apocalypse — he's definitely seen some things that would make an average person question their faith — but another part of him seems desperate to hold onto his relationship with God. Just a few episodes before, in episode 3, "Hunted," Gabriel hauntingly told another Reaper that "God isn't here anymore" just moments before killing him. Yet, being confronted with the faith of the Reaper before him made him falter. This is all part of an ongoing crisis of faith for Gabriel. Father Gabriel has had a complicated relationship with his faith since the start of the apocalypse. He clung heavily to it when he was first introduced, but after realizing that his religion enabled his cowardice, he struggled to balance his beliefs with what he needed to do to survive.
He most likely lied because he felt no one else would understand his internal battle; his relationship with religion is unique, and he is with a group who would likely only see that he failed to do what he promised. He has been the cowardly burden before, and he doesn't want to return to that. This decision could come back to haunt him and admitting he let the man live could have repercussions within the group. On top of that, it seems Gabriel himself doesn't yet fully understand why he didn't kill the Reaper. While this struggle may cause trouble for him in later episodes of The Walking Dead, it is further evidence of Gabriel's evolution.