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In the Deathly Hallows Part 2 movie, Harry was looking in the Pensieve from the vial Snape gave him and he saw Voldemort killing Lily. How could Snape have known what it looked like?
Also you could see Harry looking at Snape's Patronus in the forest. How could Snape have seen that?
In the section of the film which deals with Harry's numerous revelations while in the Penseive experiencing Snape's memories, there are some scenes which are what Harry is actually seeing at that moment (Pensieve memories) and others which are flashbacks or shots of related events.
The scene with Harry seeing the doe Patronus is a flashback to an earlier scene in the film. We see this because it's what was crossing Harry's mind at that moment: he sees the scene with Snape, Dumbledore, and the Patronus in the Pensieve, and then for a split second he recalls seeing the same Patronus himself. Because the film is following his thought processes, we share this flashback at the same moment.
As for the scene of Voldemort killing Lily, that's probably just included for drama and pathos, to make this whole section of the film more gripping. If you want it to make sense on a level different from simply "some random crap they stuck in to make the film more exciting", then maybe it's a repressed childhood memory of Harry's own! After all, he was the only other person present when Voldemort killed Lily, and therefore the only witness.
It is unclear how Snape could have known how this scene looked, however we can infer artistic license on the part of the filmmakers.
A clip of the act of Voldemort killing Lily was probably deemed to be a lot more impactful than any other way of representing the events to a film audience.
