Is it considered an enemy support card still?

I guess my question is too vague. Would smash em all remove a mindkiller/word of pain attachement played on one of my units?

Off the top of my head Smash 'em All reads "Destroy all enemy support cards" when you trigger it. Your opponent has played the support card (mind killer) onto your unit however it is still his card. He still has control of it so when Smash em All goes off it will blow up all of your support card but not any of his. Unfortunately for you Mind Killer is still a support of his so it isn't going anywhere!

Hope this helps!

Any card you put into play or played from hand is your card regardless of where it is located on the board unless a card effect was used to give control to another player.