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And it got me thinking. Unlike singleplayer RPGs, like Deus Ex, Skyrim and what not. MMORPGs never let you go the peaceful route. Its always kill kill kill, or supply equipment for people to kill kill kill.
Deus Ex, you can stealth past all content without a single death. Skyrim you don't need to fight at all (and there are play throughs of that).
However, MMOs, seem to be stuck in the primitive past of killing. And I'm sure many killers in the above link probably played MMOs, like WoW and what not. And then there is of course MMOFPS games like Planetside...those are very bloody and violent, more so than even a regular FPS.
The only genre I can think of that has no issues with training killers or making people violent are MMORTS games, but those are rare. Likewise the whole strategy genre is filled with peaceful people, that while, yes, there is killing, it isn't nearly as violent (in most cases, except Warhammer is super violent).
I think MMOs need to catch up to the times, and offer peaceful routes that have no relation to violence. And that probably means being able to stealth through content like in Deus Ex, or be able to beat quests/dungeons peacefully like in Skyrim. As they are, again, stuck in the primitive past of killing. And mass murderers thrive off this, as sort of said in that news article.