So much of Doc's character and issues are defined by the fact that his war crime was celebrated. He was called a hero for it.
It explains why he doesn't believe anyone who tells him he's a good person, because he was told that before when he murdered thousands of people.
It's why beyond his passive suicidal nature he doesn't entertain the idea that he could do more good as a vampire than a human when Owen offers to turn him. He tried to do a good thing before, was told it was a good thing, and it resulted in the deaths of thousands of people.
It's also why Pearl saying "I'm disappointed in you" is what snaps Doc out of his level three rampage/PTSD trip/grief and horror spiral, because when he was hearing those boots marching the first time and had that medal pinned to his chest no one was telling him they were disappointed in him.
Yes, the reminders of his friends and the people who needed him helped him come back, but he needed to be reminded that someone else could reel him in and tell him he's not doing the right thing.
I think this core part of Doc is also why he has such an abiding trust in Cleo. Cleo noticeably knows what it's like to think terrible thoughts and have terrible ideas because she is the one who keeps telling him that he is a strategic target to kill both because of his medical expertise and his later ability to make holy water. They call this out constantly and are not afraid to say blunt things like "you're taking this too far."
Doc more than anything is a man who has had to rebuild his moral compass from the ground up surrounded by people who would have thought he was insane for thinking he did the wrong thing. In canon we have no evidence of anyone in his past helping him figure out how to never become that kind of person again. There must have always been a part of him in the back of his head that thought that since he was both the man who committed that atrocity and also the man who rebuilt himself there was always a chance what he thought was right was actually horrendous. So it must have been a relief to find someone else who could call him out if he went too far again, even if it was the smallest chance.
He would never put the burden of his moral compass on someone else intentionally. He is a fiercely independent man who believes that he is the only one responsible for himself (even though he considers himself responsible for everyone else around him). But it would have been nice to find someone who trusted him enough to tell him if he was going down the right path.
Thankfully Pearl was able to bring him back enough with her call-out that Cleo was later able to get through to him to talk about the plan to get everyone out of Oakhurst.
It's a shame that the man who always wondered if somehow he was still a monster, managed to think dying was a good thing. Even when at the hands of his friend. One last terrible thing after all.