I just got the orientation from the DM:
So he could have stepped to the right (his left as he faced south) and avoided the Tank.
Okay and did he step backwards away from the the enemy after casting instead or did he just stand still? Because if he backed away it makes some sense if he stands still not so much.
@ MageHunter - Says who? "The best battle is a battle I win. If I die, I can no longer fight. I will fight fairly when the fight is fair, and I will strike quickly and without mercy when it is not." From Faiths of Purity part of the Paladin's Code for Sarenrae. Just because you're good doesn't mean you don't kill people when you feel like the situation warrants it.
Someone up thread said that in the situation there is a reason the women are attacking the PCs but the OP hasn't told us if the characters know that reason yet. As such all they know is that these people have jumped them before and tried to kill them, they're enemies there isn't any reason to keep them alive unless you want to interrogate them.
The OP also said that the party told the sneaky person to "go deal with them" "deal with them" is very commonly used as a euphemism to kill somebody, without further explanation it would be reasonable for the character to assume they wanted the potential threat dead.