Okay, fine.
Given how so much of the Equestrian universe is tied to emotions, I would think that 'dark' magic is primarily tied to 'dark' emotions. That is, the problem isn't be capable of channeling anger/rage/etc into magic, but doing so on a regular basis. We aren't any more likely to see Twilight turning evil through one use of a spell than seeing Celestia do so for the same reason.
I'm not sure Sombra was any more dead after the episode than he was beforehand. It seems more likely he was just banished, or pushed out into the frozen tundra.
I've only seen pieces of previous series' movies, so I really wouldn't be able to compare. I highly doubt we'll be seeing eugenics on the show (what movie was ever THAT dark?!) and any death of a character will likely be a sad thing, not scary.