Do Destroyed cards go into the discard pile? There was an opinion at our last game that "Destroy" meant put off to the side, not in discard, never to enter the game again. I didn't think so but would like confirmation.

Destroyed or sacrificed cards go to the discard pile. As of yet (as far as I can recall from Arkham Edition forward) there are no cards that remove anything from the game.

The easiest proof I can find is in the rules .pdf ( www.fantasyflightgames.com/ffg_content/coc/support/coc_lcg_rulebook_en.pdf )on page 9 under Combat Struggle:

"Most characters are destroyed (go to the discard pile)..."

And under further examination, page 14 under Destroy and Sacrifice :

"When a card is destroyed (this includes a character that has taken a
fatal number of wounds), it is placed into the discard pile.
A character that is sacrificed is also placed in the discard pile. A
player can only sacrifice cards that he controls, that is, a player can
never sacrifice an opponent’s card. Also note that “sacrifice” and
“destroy” are not interchangeable terms: A card that is destroyed is
not sacrificed, and vice versa."

Yup. sacrifice and destroy both usually cause cards to go to the discard pile. There are some differences between sacrifice and destroy. And (for instance) responses that trigger on 'Destroyed' won't trigger when something is 'Sacrificed' and vice versa. There is no 'removed from the game' or 'exile' zone. The only way to make sure something will never come back into play is to make it a domain card, which is nigh impossible to pull off right now. And maybe one day that'll be reversable too.