The New Zealand shooter has really struck me. I know the machinery that spawned him well. I have wandered many of the same parts of the internet. I've seen the things this person claimed as himself. As large as these cesspools are, it's unlikely but not impossible that I've read this person's words in the anonymous mess. As I mostly wander as an observer, it's quantum shifting through a wall unlikely we had spoken. But still. One timeline out of an infinity is unnerving.
Six degrees of freedom. Suppose everyone on the planet is a maximum of six hops away in any timeline. Other factors influence the likelihood of timelines one to five away. The furthest being the least in your control: Fate. Zero being yours to choose as you will. If you commit an act, you force at least one timeline where that happened. Those in your social circle enforce a layer. Those at six hops enforce the cosmic background timeline. The baseline.
Moving among the nodes grants different choices to your life. The choices you're presented will be the possible timelines others experience. The one you choose increases the likelihood that other sapience will experience the same. As no relationship can share 100% experience, there is never a relationship with 100% shared experience. There is never 100% guarantee in any relationship that the experienced timelines will be the same.
The above (aided by cognitive biases and restricted communication) is why comparing memories in disagreements is often pointless. What is remembered is remembered for at least slightly different reasons and explained by at least slightly different stories. Comparing the cliffnotes of two journals is guaranteed to be hilarious.