Well to answer this question, you should really understand what all of the different keys on Steem do. Let's quickly run through them.
Posting Key - The posting key allows you to upvote, comment, post and follow.
Owner Key - The owner key can do everything the posting key can do as well as wells transfer funds, vote for witnesses and change all of your keys (this includes the owner key).
Memo Key - The memo key is solely used to encrypt and decrypt memos, this is the only key that can do this.
Active Key - The active key allows for everything the owner key can do except for changing the owner key.
So to answer the question what would happen if hackers got your active key, they would be able to transfer out your funds, vote for anyone they wish, comment and post as they please and overall cause mayhem to your account.
In the scenario that your active key is compromised, I would recommend that you change your master password.
If hackers get your steemit active key then they can use it to withdraw money from your steemit wallet and they
can also do other transactions on it,they can also use it to create contents on your steemit account without
your permission,infact the list is endless so that is why as a steemit user you need to ensure that you keep your
password safe and do not let hackers have access to your steemit account...