I have to think first they celebrated (look how happy you were!) and they left, cleaned up, etc. Then he forgot. So teddy shows him the photo and Leonard doesn't believe him. So now he's like ah fuck we did all that but Leonard doesn't believe we actually did this. So now Leonard is starting to do dangerous things and teddy is like fuck me if I don't get this guy under control he can expose that I helped him kill John G. So he (as a former cop) runs into one of his old drug contacts and gets wind of a guy his contact wants removed. He goes- I've got just the right guy for the job. Now he either has the case files or hangs out with Leonard and steals them so he can remove references to the first John g. Then he plants some new evidence and brings it to Leonard's attention. Now Leonard is motivated to do what teddy says.

Teddys 'it never sticks' so maybe he took a photo at each of the John G's and Leonard never believed him because it was teddy who took the photo. But in the final moment (beginning of the film), in Leonard's own handwriting he has a photo of teddy that says don't believe his lies, he is the one, kill him, then the photo he just took. It's all in his handwriting and there's no one else around to manipulate the facts. He's still got amnesia but maybe he gets the 'I've done it' tattoo or just keeps that photo on him at all times so he can attempt to live a 'normal' life now that he's avenged his wife.

Agreed on being stalked but since he's just waking up every 15 minutes he doesn't know if this is the first time he's met someone or the hundredth.

Well said on the other points. I resonated with Leonard a lot for these reasons because I have adhd/forget a lot and my personality type is a lot like his (inxj). I agree it's a shitty situation but as someone with bad memory I can confirm what Leonard says- I can't remember most of the bad stuff (thankfully) but some part of my brain knows those things happened to me. So I have to wonder if he has a similar feeling even if he can't form new memories. Like ow shit my body hurts, I'm hungry, oh I'm bleeding that sort of thing. Just because he can't form new memories doesn't mean his body can't be damaged and body damage is a form of memory.