Oh good observation! I didn't actually accounted the role of the runes either haha. Though, that sounds absolutely fitting. More like the theme setter for the games eh?
Yes, Riou had the Bright Shield as you pointed out, so he must be all goody-goody to an extent, I guess. However, I think that shouldn't stop him from having executioner decisions though, considering he is already a leader. I dunno maybe he's the type who is raised by morale masters like Viktor and Flik or whatever lol, that's why he behaves like that. I guess all the destructive traits were designed as an embodiment of Jowy lol. It's more like a Yin-Yang kind of relationship imo.
Lazlo on the other-hand is more like a "shock" victim for the Rune of Punishment since he has no actual good or evil "trait" in the beginning. Seems like a slave of Snowe even in the events from Tactics. But I guess this makes his decisions of execution unquestionable because he's neutral. (and I don't think he really suffered from the rune either... *smirks*). I won't be bugged as well if Snowe is executed since he's a jerk from the beginning and is obviously just a pretender who keeps using people for his sh*t.
I haven't really touched S5 so I won't have a word on it but since you mentioned it, I guess it just makes sense.