I'm much more a fan of the Bruce, than I am of Wallace. The Bruce was one of the top ranked knights in Christendom, and though he may not have been perfect, most accounts of his martial actions would indicate that he's anything but a coward... it takes b@lls to stab your enemy to death in a church, wouldn't you say?
Bruce wounded him, but his cronies Lindsey and Kirkpatrick finished the deed ("I'll mak siccar!"). I would say it was less b@lls and more anger and haste that caused Bruce to attack Comyn.

I've always wondered what Bruce & the Comyn could have achieved had they put their differences aside and worked together, but it was not to be.

T.