If I activate Baited Hook on my Unsung Hero, can my opponent react and kill it first thus making my Baited Hook do nothing?

Yes, your opponent can react to your Baited Hook activation and remove the targeted unit before your ability resolves.

According to RiftJudge FAQ #4379, if your opponent removes the targeted unit in response to the activation of Baited Hook, the following occurs:

1. Activation: You activate Baited Hook and declare your Unsung Hero as the target.
2. Reaction: Your opponent plays a card to remove/kill Unsung Hero.
3. Resolution: When Baited Hook resolves, its "Kill a friendly unit" instruction cannot be followed because the target is no longer on the board. Because the unit was not actually "killed" (it did not go from the board to the trash via the ability's instruction), the reference to "the killed unit" returns a null value for its Might.
4. Effect: You will still look at the top 5 cards of your main deck, but you will be unable to play any of them (as you cannot satisfy the "Might up to 1 more than the killed unit" requirement), and all 5 will be recycled.

In short: Baited Hook will still resolve, but you will not be able to play a new unit.