Not sure if it's a bug or design

When I hold my mouse button down on a mob I have to keep clicking it every time a mob dies or it just makes me run around instead of attacking.

Example:

Surrounded by 10 monsters, I click and hold down my mouse button to keep swinging at him until he dies. Once he dies, character begins running. I still have mouse button down and hover over another monster but character won't attack. I have to lift MB and click again to initiate attack. However it can be tricky because my char is still running around so the target is harder to click on.