I can't believe there's even a debate.
Regardless of where it came from, what media it is on, who it was from, stolen or not there will always be the issue of authenticity.
Other than that, it should be taken seriously for evidence, after all if the event actually happened and there's suddenly new evidence, leaked or stolen or not, you could potentially help justice. Then again, if it's fake, corrupt justice.
But even "legally obtained" evidence could end up being from a fake source.
It shouldn't automatically be dismissable by the means of access, once it's seen whatever punishement follows should be seperate from the subject case on the video/print/etc.
I always use this when talking to people about anything releated to murders/law, imagine it happened to you, or someone close. It's too easy arguing when it's just an imaginary example or unknown person you don't care about.
But if the threads OP happened in real life and it was your closest person in your life, I don't think you'd have much problem of how it came to light, just authenticity, always that! It has to real and that's hard to prove sometimes.
I remember this was debated in my 6th form's debating society and caused quite a stir. The room was divided. People who studied A Level politics and law sided with the evidence not being allowed to be used. Others sided with it being allowed.
No surprise, there are parts of law, so ingrained and passed down from elder to younger that they have no possible understanding of what's going wrong, just because many life times ago a group of humans decide this is right and this is wrong, doesn't mean it should still exist. It's sad, you'll have people who will guard and defend all the red tape as if it were their own child, they'll even use it to get around systems, or to get someone in trouble out of it due to technicalities, law is very complex but it isn't perfect either, not much is perfect. Someones life was taken and there's probable evidence, to dismiss it on a whim is a bit naive and childish to jump to.
You must always question, or we give in.