a few reasons why pat might not have killed the kid in real life?

Apparently Garrett had the body taken away after he had supposedly killed Billy and had him buried without anyone witnessing that it was in fact Billy. Normal procedure would have been for someone to identify the body other than the man who killed him (Garrett). So the fact that Garrett acted suspiciously regarding showing the body definitely raised a few eyebrows.

The problem is that hat all of that is hearsay. Even from Pat Garret, he is own reflections are dubious at best. However, they remain the only primary source about The Kid in existence.

I then concluded to go and have a talk with Peter Maxwell, Esq., in whom I felt sure I could rely. We had ridden to within a short distance of Maxwell's grounds when we found a man in camp and stopped. To Poe's great surprise, he recognized in the camper an old friend and former partner, in Texas, named Jacobs. We unsaddled here, got some coffee, and, on foot, entered an orchard which runs from this point down to a row of old buildings, some of them occupied by Mexicans, not more than sixty yards from Maxwell's house. We approached these houses cautiously, and when within earshot, heard the sound of voices conversing in Spanish. We concealed ourselves quickly and listened; but the distance was too great to hear words, or even distinguish voices. Soon a man arose from the ground, in full view, but too far away to recognize. He wore a broad-brimmed hat, a dark vest and pants, and was in his shirtsleeves. With a few words, which fell like a murmur on our ears, he went to the fence, jumped it, and walked down towards Maxwell's house.

Little as we then suspected it, this man was the Kid. We learned, subsequently, that, when he left his companions that night, he went to the house of a Mexican friend, pulled off his hat and boots, threw himself on a bed, and commenced reading a newspaper. He soon, however, hailed his friend, who was sleeping in the room, told him to get up and make some coffee, adding: 'Give me a butcher knife and I will go over to Pete's and get some beef; I'm hungry.' The Mexican arose, handed him the knife, and the Kid, hatless and in his stocking-feet, started to Maxwell's, which was but a few steps distant.

When the Kid, by me unrecognized, left the orchard, I motioned to my companions, and we cautiously retreated a short distance, and, to avoid the persons whom we had heard at the houses, took another route, approaching Maxwell's house from the opposite direction. When we reached the porch in front of the building, I left Poe and McKinney at the end of the porch, about twenty feet from the door of Pete's room, and went in. It was near midnight and Pete was in bed. I walked to the head of the bed and sat down on it, beside him, near the pillow. I asked him as to the whereabouts of the Kid. He said that the Kid had certainly been about, but he did not know whether he had left or not. At that moment a man sprang quickly into the door, looking back, and called twice in Spanish, 'Who comes there?' No one replied and he came on in. He was bareheaded. From his step I could perceive he was either barefooted or in his stocking-feet, and held a revolver in his right hand and a butcher knife in his left.

He came directly towards me. Before he reached the bed, I whispered: 'Who is it, Pete?' but received no reply for a moment. It struck me that it might be Pete's brother-in-law, Manuel Abreu, who had seen Poe and McKinney, and wanted to know their business. The intruder came close to me, leaned both hands on the bed, his right hand almost touching my knee, and asked, in a low tone: -'Who are they Pete?' -at the same instant Maxwell whispered to me. 'That's him!' Simultaneously the Kid must have seen, or felt, the presence of a third person at the head of the bed. He raised quickly his pistol, a self-cocker, within a foot of my breast. Retreating rapidly across the room he cried: 'Quien es? Quien es?' 'Who's that? Who's that?') All this occurred in a moment. Quickly as possible I drew my revolver and fired, threw my body aside, and fired again. The second shot was useless; the Kid fell dead. He never spoke. A struggle or two, a little strangling sound as he gasped for breath, and the Kid was with his many victims.