Wouldn’t hurt a fly = totally harmless and would never hurt anyone.

• You needn’t to be afraid of him.  He wouldn’t hurt a fly.

Give someone free rein = give someone complete freedom.( reins are what you hold in your hands to control a horse when riding)

• They’ve given me free rein to do whatever I like in this new job.

Make a beeline for = go quickly and directly to.

• As soon the meeting was over, we all made a beeline for the food.