Spoiler: Example of Real adventure we've done
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I can recall vividly a set of adventures in the past year where we were colonists of sorts in a new land that was relatively unsettled aside from a few indigenous elves in a nearby forest. As opposed to a usual adventure, there was a period of 2-3 years (in game time) over ~3 sessions, where the sessions revolved entirely around building up the town, and settling the surrounding lands, Age of Empires style.
Instead of actually helping with these activities, for the first year or so, our parties Cleric of "Wee Jas", who we later found out was actually a LE worshipper of Asmodeus, slowly was corrupting the entire town, constantly passing notes in secret to the DM, and starting a cult.
Once we got the town up and running, the players decided to host a great celebration, Thanksgiving style, like in the old days. The Cleric volunteered to take responsibility in arranging the meal and preparing everything, reasoning that he hasn't done much in the past and as his way of making ammends. We all agreed, and went on a small tour of the surrounding lands to fully appraise what all we have done.
A few days later, the meal is about to take place, the Cleric has made what appears to be a wonderful display of a meal, with every food imaginable for the times, and several types of ale, wine, etc. All of the townsfolk, not affiliated/dominated with his "cult" and all of the PCs except him dig in and start having a great time.
This Cleric begins giving a speech you would expect for such an occasion, but in actuality, is using Silent Spell to cast a "ritual spell" he has arranged in secret with the DM, to sacrifice such a large number to Asmodeus, for being "willing" since, he argued, they willingly ate the food.
So Ritual complete, everyone who was eating the food, PCs included, suddenly burst into Hellfire or something, die, and have their souls forever imprisoned in Asmodeus's gallery of playthings. The Duke of Hell even makes an appearance on the Material Plane after this little stunt, and grants the Cleric some ungodly number of Wishes/boons due to sacrificing somewhere near 500 people at once.
Suffice it to say, campaign was over, and the Cleric became the next BBEG for our Epic Level Campaign, since he went from level ~8 to somewhere in the mid 20s from all of his wishes, hooray DM fiat for Wish wording.
This is the type of shennanigans I would like to avoid. Normally I would have never of had a problem with the above circumstances if they would have either played out over the course of months of actual play time, or we were actually going for an evil campaign. The entire basis of that adventure was "You are goodly people loyal to the crown being paid to venture to unknown lands and settle them for the Crown" and one guy had to ruin the entire thing cause he felt like being a head cultist/"satan" worshipper.