Why, near the end of the 2012 movie "The Hunger Games", first Peeta wants Katniss to kill him, then stops her from apparently killing herself with the berries?

To rephrase, what happens when a person tries to kill itself in the hunger games?

Peetas whole job was to protect Karniss. He was disposable until he needed to protect her.

It makes sense in the book. Don’t remember how it translated to screen, but I vaguely recall being annoyed at how the ending was adapted.

From what I recall (and just in case anyone hasn’t read/seen it happens to be reading this, SPOILER WARNING)—Peeta loved Katniss and knew he wouldn’t kill her, so had been expecting to die in order for Katniss to live. Katniss refused to kill him, but didn’t see a version of the ending where both of them would walk out alive, and wanted to give a big fuck you to the game makers by depriving them of a victor at all—so she and Peeta decided they’d kill themselves together, simultaneously. When they put the berries in their mouths, the game makers announced they were both victors, because having the final two contestants commit suicide together would be bad for the game’s image. And so they both got to live.

That’s what I recall, anyway. Read it a while ago. (I think that answers your rephrased question too?)

Peeta deeply loved Katniss and when the Games Maker, Seneca Crane, reversed the earlier rule he made allowing two ‘Players’ to live instead of the normal one victor provided that both came from the same district Peeta told Katniss to kill him so she could be the sole survivor.

However Katniss realised that there HAD to be a winner so she picked up the berries and gave some to Peeta and told Peeta that they should die together rather than give Crane the satisfaction of having a winner. Peeta did not realise Katniss knew that Crane would not allow both of them to die and that is why stopped Katniss and Peeta from eating the berries.

She was obviously right because as soon as they both had counted to three and started raising the berries to their mouths the Crane quickly pronounced both as winners.

The great thing is that Snow hated what the Crane did and he was taken to a room with the same type of berry that Katniss was going to eat and he knew he was expected to eat them and die, which he obviously did.

Poseidon