[EDIT] I tested this in tactical and it seems the 'autoattacking' is a separate issue from what i wrote below. I saw the auto attack happen with a wolf with deadly bite ready, i could not move next to an enemy then use deadly bite because after moving the wolf always attacked normally before i could select the ability, even after i made sure the highlighted tile is the one next to target and not under him.

I believe this is caused by an annoying habit the cursor has to highlight a different square than the one it is placed over. It is not limited to tactical combat either, happens a lot on the world map as well. I first noticed it with version 1.0.

Basically what happens is you place cursor over the middle of a square, the square is highlighted as movement target, then you move your cursor out of the square and over another, but as you cross the square border, the game still shows the old square selected as movement target, despite the cursor already being over another square, and does not switch movement target to the new square until cursor is over the middle of the new square.

So, if in tactical, you have a hero selected, mouse over the enemy beast, then move the mouse to the square next to the beast but not far enough away from the beast's square, then click, the game will receive the click while the beast's square was still the target, making the hero attack it. Only way to avoid it is paying close attention to what square is highlighted before clicking.