KrystalS, we're having the same problem (Mac Pro, also running OS 10.5.8), so it's not something YOU did. Unfortunately, I do not know the solution. Hopefully, the fact that you're not alone and that we're running the same OS will provide more info so someone can help us both.
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http://www.minecraftforum.net/topic/1867928-minecraft-16-launcher-game-ended-with-bad-state-exit-code-139/
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