I am pretty sure this effect will trigger after creatures have been put into a graveyard, since for the ability to proc reads “when it kills a minion” meaning after damage has been resolved. At least that is how I am reading it, I might be wrong.

If this is the case, then it’s pretty bad. If I am wrong however it’s an ok card.

At least Lorewalker Cho is fun.

At 4 cost its not even viable to pull of combos itself, though it is clearly meant for decks using either Gadgetzan Auctioneer or Combo cards, due to the coin.
– You will be using 4 mana, that might be all you get to do that turn, early on. Even if you remove a minion with it you are basically handing your opponent the turn in exchange for a coin..

If it had been common, perhaps this would have been acceptable, but as an epic with that cost and limitation, this might just rank amongst the worst epics ever. I would not spend 400 to craft this card.
– Had it been 3 cost, do 3 damage, minion or player, and gain a coin every time –
Like this? Worst rogue card in a long, long time.