I was thinking the same thing when Walken and several of the security detail draw their weapons right as Depp is about to take the shot at the governor. It makes sense in that, in case Depp messed it up or missed or whatever (it's not like he has any experience with guns as far as anyone knows), they could take her out and let Depp take the fall for it. What I don't understand, though, is that a forensic investigation would likely reveal the multiple weapons potentially used.

the security detail was in on the plot to kill the governor, why wouldn't one of them do it?
It looked to me like that's what they were going to do. Depp was a diversion to cover for the hitter who was in an overlooking room. That's the man who shot at the governor but put the bullets instead into the security man's kevlar vest. Depp then fired a couple rounds at the shooter.

If so much of the security detail was in on the plot to kill the governor, why wouldn't one of them do it?
Hmmmm, I dunnomaybe because they would go to jail?
And if they wanted an outsider, a gun-for-hire wouldn't have been hard to find.
The governor's husband suggested that, but it would cost money and it could be traced back to the husband and his wife's security detail. On the other hand, a guy alleging "strange people in suits blackmailed me" would be a hard claim to prove. Especially since this same guy was caught on camera looking crazy and confused.
Why involve an innocent man and his daughter?
Because a caught off-guard innocent man would have more motivation if his daughter's life were at stake, compared to what $10,000 is to an experienced killer.
I understand that Walken's character is psychoticbut the other conspirators would sense this about him also.
Your point? Walken's character seemed to be a mercenary/ex-marine of some type, he was an expert at interrogation, defensive fighting skills and deliberate cruelty to get the results he wanted. The whole movie was more a battle of wits between Depp & Walken's characters than a suspense film - Depp thinks he can find a way out and rescue his daughter, and Walken kept him in check and told him he had no place to go. There's not a single fistfight between both characters in which Depp's character was victorious - however, by the time he got out of Walken's sight (a strategy which WAS risky and suspenseful), he was one step ahead of the conspirators.