ppl are like "why didn't they just shove a criminal slated for execution into the veil to close it?" and they're right. it would be kinda dishonourable, i guess, but you're gonna kill them anyway so . however, this problem could be avoided entirely by canon simply stating that the callieach demands a willing sacrifice, the same way that gwen had to enter the lake of her own accord in 5x09. according to kilgharrah, the veil doesn't ask for anything - the callieach is the one who demands a soul. maybe she'd just take any soul, but she kinda seems like the type who wants to be entertained. she wants to watch people trip over themselves to die, to betray each other and show their 'true faces'. it would work well and patch a plot hole considering arthur, merlin, and lancelot - who set out to sacrifice themselves - were all willing to enter the veil.