It can be as simple as he kills Logan to protect Dolores. He'd see Logan every day of his life in his wife, Logan's sister.
Here's another thought. MIB talks about how he saved the park 35 years ago from Arnold trying to destroy it. Well, Arnold is trying to destroy it posthumously through his creations. In particular, Dolores (since we now know it was Arnold talking to her, not Bernard). The fastest way to kill it would be to sully its reputation (massacre) and kill the (remaining) creator (Ford). So maybe he saves Westworld by killing Dolores to save Ford (35 years ago). And that messes him up. Over the last 35 years, he realizes he made the wrong choice and is coming back to finish Arnold's job (destroying the park/tech).
So, I'm not really disagreeing with you. Just having fun running out tangents and talking about it with others.
Now here is a thought in the Logan direction. Westworld employees have talked about "the real purpose" of the hosts. Now when you have artificial intelligence robots, the first go to is "military uses". That is a common theme in these types of stories. But I really don't see any lead up to that. Maybe it isn't about just rich people having a unique vacation. Maybe this is actually a unique therapy.
People that have the urge to kill, but haven't actually done so can come to Westworld and hopefully release the building tension.
People that have rape fantasies can indulge them without harming "real people".
And in Logan's case, he's knows (but doesn't admit to others) that he's emotionally messed up. And his shrink suggested he find himself in Westworld. He becomes part owner so that he can go beyond the standard guest in terms of privileges. He has some small degree of success with his visits. He isn't fixed, but it takes away some frustration. His sister is marrying somebody he likes, but sees him struggling with a different problem (assertiveness, standing up for himself, etc, almost the reverse of himself). He might not care enough to help, but this is his future brother-in-law, so he's going to fix him. He brings him to Westworld hoping it will help William the way it helped him. But maybe he helps save the park by killing William. That was a real person. So now Westworld is ruined for him and he can never go back. When he goes back to real life (and his family) his terror tendencies build up and reveals itself again to the "real people" he loves (but can't truly care for). That's why his daughter had to tell him he caused the wife's suicide. He is such a psychopath, he doesn't even put it together until somebody else he "loves" tells him. So back to Westworld as MIB.
Honestly, I think both can be done in a very compelling way. Or we are both wrong and maybe the real answer will be even better.