My assumption is roughly the opposite - the intent of the video was to prejudice the jury as far as possible against the accused (he killed a person honorable enough and empathetic enough to suggest a more lenient sentence! To the pit with him!)

It wasn't about the content of the video - it was about the emotional response to an AI generated facsimile of the victim.

It eventually makes sense to my brain, aside from the AI part of course. If I'm a remarkably forgiving person my family might both appreciate that about me, and not be remarkably forgiving themselves. And they might say look, we lost this remarkable human being, punish the responsible party harshly.

Ab-so-fucking-lutely. This paints the victim in the most positive light. And makes killing them seem even worse.

Again, attorneys understand that a lot of times you're looking for an emotional hook, not straight up "facts". This video made the victim seem moral, empathetic, full of forgiveness... as close to freaking jesus as you can get.

And that other person killed them.

Now go sentence that person.

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Instead imagine the video was of the victim smoking a cigar, laughing, and suggesting that he'd never forgive him and he should be put to death?

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Which video makes the jury more likely to actually come back with a tougher sentence? The one where he's a bastard, or the one where he's a saint?