The Devastating Truth Of Deadpool 2's Ending

3. Time Travel And Causality

Fundamentally, time travel has to abide by certain rules or it all goes to Hell pretty quickly. Even X-Men movies are subject to these rules, even though they've mostly spent the past few years YOLOing their way through various timelines without a care in the world.

The rules are complex and some will make everyone's brain hurt, but the simple (and important) ones are thus: you can't change anything in the present or the past without there being causality in the future. And you cannot cause an event in the past from the future (which Interstellar famously forgot).

On top of that you have your special rules like the dangers of meeting past or future versions of oneself or having to travel naked (as in The Terminator), but they're more open to interpretation. The ones about causality and the ramifications of your actions cannot be argued with for very simple reasons.

And in order to accept Deadpool 2 has a happy ending, you have to ignore one very simple truth: that there is absolutely no way Cable could possibly have changed the future. His family MUST still die - as heartbreaking as it sounds - and Firefist must still kill them. Because, otherwise, he wouldn't continue to exist in the present...