Why send Matthew to kill her?

So obviously she's got to survive to take revenge, but With the other thre guys wanting her dead they send the guy who's objecting to killing her and didn't guess he might do what Matthew ended up doing and not stab her but dip the blade in blood that she'd already bled?

I'd say there were a couple of different options for that part of the film.
Either skip that scene entirely. They just leave and get on the boat, no talk of killing. Wiouldn't exactly raise the audience's sympathy with them after everything they'd done.
Or they could have sent Andy, he gets a bit of conscience and does what Matthew did. Matthew was objecting to it, Johnny was the one who came up with the idea to kill her, Stanley looked like he was trying to kill her before Andy pulled him off. That leaves Andy as one that seemed to agree about killing her but by default would be the least likely to go through with it. After all it was him who pulled Stanley off. They would probably have trusted him to do it, but maybe he could have second thoughts and just get blood on the knife and leave.