It was Sen. Henry who paid the gunmen to kill Sen. Benson, but it was all too obvious. He knew from the onset that he would be suspected and investigation just might reveal him.

So to avoid this he was going to make himself look innocent.
But how?

He forged a brilliant idea:
On the same night he sends men to Sen Benson's house he was going to send men to his own house too. The men were going to make it look like he was also attacked, kill all his night guards and injure him slightly with a bullet.

So that same night, during the fake attack: When Sen Henry heard the noises (although scared) he knew the fake attack was ongoing, but then he heard a gunshot and was surprised, because firstly, he had secretly unloaded his guards' guns earlier on (to ease the fake attack) , secondly, he knew the intruders' guns had silencers.

Then he started to think it might be a real attack from another enemy. So in panic he went out to confirm. While out he was mistook for a guard and shot at, he ducked, jumped off the fence, and was pursued..

@nevies