OK, I understand that they're basically a stand-in for the Talon Company from Fallout 3.

That being said, Talon Company had been hired to kill the Lone Wanderer, so their actions in Fallout 3 were understandable.

I can't understand why the Gunners in Fallout 4 want to shoot everybody on sight. If they're mercenaries I ought to either be able to hire them or maybe even work for them.

How do they know I'm not a prospective customer walking up to their camp? Shooting me could be bad for business.

They seem to be little more than Raiders with better gear.

Go to Quincy and read Clint's terminals.

The Gunners are working to take over the Capital Wasteland after years of being raiders.

It's why they destroyed the Minutemen and hate anyone involved with them/trying to rebuild them.

They also do raiding.