It's clear to me why Parisse got the red card. I suspect it's clear to everyone .. it's because he deliberately raised his elbow into position to make contact with the head throat area.
Yes, he was taking advantage of the poor posture and great speed of the arriving player , but it was deliberate. To pretend otherwise is disingenuous
Does it deserve a red card ? That's a valid question, but we need to discuss what actually happened .. the deliberate contact with elbow/forearm
This was NOT a rugby accident caused solely by poor technique
If the elbow had been held away from the body or even put in place fractionally before the impact I would have some sympathy/empathy with your position. However his elbow was there in plenty of time before the impact and was only moved away afterwards for the reasons I said above.
We allow ball carriers to run into tacklers with their elbow/shoulder tucked in don't we? And expect the tackler to tackle correctly accordingly? If we don't then what game are you observing/playing/officiating?