Sorry you had the bad experience but we've all been there. 8 inches of penetration is enough to kill but as BC said, 8 inches of blood doesnt necessarily mean 8 inches of penetration.
It sounds like you may have gotten substantial bone and one lung involved.

Thats a bad combination but the deer may revover. It will depend on how the one lung that was hit.

As for mechanicals, I also am not fond of them. They have come a long way from the original punchcutter but the problem lies more in their misusue that anything else. Far too may guys use them as a bandaid on a poorly tuned bow. The loss of kinetic energy due to poor arrow flight and the additional loss when the unstable arrow impacts the animal are ignored by too may hunters. Use the mechanicals if you want, but please make sure that they are not on your arrows simply because you cant make a fixed head group right!