Its not instakill. The damage isn't the problem. I agree that my suggested change will still mean rune prison still sucks (mostly though it will help SOMETIMES) for melees, but that's beside the point.
Snares/roots are always going to suck for melee, plan your build/playstayle aware of that, get banshee's veil or MR items. I'm not complaining about rune prison because of its relatively high damage or how rune prison is imba, just how it's not what it says it is and how it SHOULD be, according to the description.
Let us attack from within!