She has been brainwashed
From "Let's Kill Hitler" (S06E08):
MELODY: Well, now, enough of all that! (pulls a gun and aims It at the
DOCTOR) Down to business.
DOCTOR: Oh, hello. I thought we were getting married.
MELODY: (walks towards them) I told you, I'm not a wedding person.
RORY: Doctor, what's she doing?
DOCTOR: What she's programmed to.
RORY: Where'd she get the gun?
DOCTOR: Hello, Benjamin.
She was raised by the Silence, whose specialty was brainwashing people (that's how that got humanity to go to space, and ultimately the cause of their downfall). Besides being brainwashed, she was also raised to believe that the Doctor deserved to be killed for failing humanity.
From the same episode:
MELS: A significant factor in Hitler's rise to power was the fact that
the Doctor didn't stop him.
Why did they put her in a suit?
That wasn't necessarily their only plan. They wanted a part-Time Lady assassin, who would have the skills necessary to take down the Doctor. Indeed, River previously tried to kill the Doctor with poison (and was very nearly successful at it). She might have benefited from her skills as a Time Lady (for example, regeneration) in doing so. The suit may have been a backup, in case she wasn't able to kill the Doctor of her own volition.
It's pretty unclear why they couldn't simply have put someone else in a suit in that case. Maybe the suit was only compatible with Time Lord physiology. Or perhaps it was because "destiny says so," and River simply had to be the one to (apparently) kill the Doctor, one way or the other.