All time travel stories require varying degrees of suspension of disbelief. The only fully self consistent plots involve "closed loop" stories, like Predestination, but those require us to give up on the concept of free will. Some are just incredibly stupid (Looper comes to mind). On the scale of things, Bill and Ted's Excellent Adventure isn't all that bad.
The good thing about this movie, is that it puts the really implausible stuff right at the beginning, and once it establishes the rules, it mostly sticks to them.
Two randos build a (sort of) time machine out of what looks like a Commodore 64 and a pile of literal junk (seriously, this movie gets a solid F for prop design). Rather than send people back in time, it has one and only one capability: it allows the users to kill a specific person in the past, as long as they can locate that person at a specific time, and of course this tees up a classic morality dilemma.
This changes the present and everyone in it, but because "yada yada yada quantum mechanics", anyone in the room with the machine will remember the original timeline - and NOT have any of the memories they should have in the new timeline. This is actually an issue for a lot of time travel movies, they generally just sweep under the rug rather than confronting it outright.
They start with a pretty clear case where killing a bad person in the past will save the life of a innocent person, but it won't be a spoiler to tell you that this has unintended consequences and that their attempts to set things right will just make things worse, or at least put them further and further from their original world.
So in the end, it's a mix of the butterfly effect, the multiverse, and the Trolley Problem. In spite of some of the other reviews, I found the acting and emotions pretty good. I think it would have made a good Twilight Zone or Black Mirror Episode, but stretching it to a full length movie got a little thin.
I didn't hate the ending, but a lot of people did.
